Using Simulation to Understand Bottlenecks, Delay Accumulation, and Rail Network Flow

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1 Using Simulation to Understand Bottlenecks, Delay Accumulation, and Rail Network Flow Michael K. Williams, PE, MBA Manager Industrial Engineering Norfolk Southern Corporation, 1200 Peachtree St., NE, Atlanta, GA / michael.williams@nscorp.com Word count = 3, ABSTRACT Many railroads today use simulation models to analyze line capacity and quantify the benefits of adding infrastructure and improving operating procedures. Typically, the modeler will simulate various improvement scenarios and measure the reduction in total network delay or, stated as a normalized value, delay per 100 train miles. This is a reasonable approach, but relying on this static metric alone may lead a modeler to make suboptimal infrastructure investment decisions. A bottleneck is a restriction of flow that leads to delay, but delay is not a homogenous value that applies equally across a rail network; delay accumulates at specific locations. Rail network delays are part of a system and each delay is influenced by the others in a dynamic environment of mutual-dependency. Changing the delay at one location will reduce (or increase) the delay at all other locations. It is possible to reduce total system delay while at the same time increasing the congestion, and hence the risk of a service failure, at another location AREMA

2 Using simulation examples, this paper offers an innovative approach to allow modelers to better see where rail network delays are accumulating and understand the relationship between bottlenecks, delay accumulation, and rail network flow. By analyzing a network s delay zones, a modeler will be able to pinpoint the delays, determine their magnitude, and measure how well each responds to new improvement scenarios. With this knowledge, the modeler will be able to make more informed capacity investment decisions. INTRODUCTION Many railroads today use simulation models to evaluate line capacity and quantify the benefits of adding infrastructure and improving operating procedures. Modelers will typically simulate various improvement scenarios and measure the reduction in total network delay or, stated as a normalized value, delay per 100 train miles. This metric provides a high-level view of the health of a network and allows a modeler to compare competing scenarios. This is a reasonable approach, but this metric does not tell the whole story, and making decisions based on this single value may lead to suboptimal infrastructure investment decisions. Before recommending new capital projects, modelers should know where delay is accumulating and understand the relationship between bottlenecks, delay accumulation, and network flow. One tool that can help provide the necessary insight is a delay zone graph. This paper does not provide guidance on how to identify specific capacity-enhancing projects. It is expected that the modeler will use experience and sound engineering judgment to identify appropriate projects that are physically and technologically feasible. The methods presented herein will, however, help the modeler evaluate the effectiveness of these opportunities AREMA

3 STARTING WITH THE BASICS Consider the case of two rail yards connected by sixty miles of single track as indicated in Figure 1. There are no sidings and the track has zero grade and a uniform speed profile. Assume there are fifteen eastbound trains and fifteen westbound trains spread throughout the day. Figure 1: Sample Network Layout Through simulation, the normalized delay for this theoretical network is measured to be 159 minutes per 100 train miles (159 min / 100 TM). However, delay is not a homogenous value that applies equally across an entire network; delay accumulates at specific locations. A modeler can see these delays by dividing the network into smaller zones and graphing the accumulated delay at each. As indicated in Figure 2, all of the delay in this simple rail network occurs at the yards. This makes sense because trains are held at the originating yards until the track ahead is clear. For example, if an eastbound train is en route, a westbound train cannot leave (it is delayed) until the eastbound train arrives. Westbound trains are delayed at East Yard and eastbound trains are delayed at West Yard. There are no delays between the yards because trains do not stop once they occupy the main track. Figure 2 is a simple delay zone graph AREMA

4 Figure 2: Delay Zones for Sample Network A traffic flow line through the delay ordinates characterizes the wait, wait, hurry up nature of the traffic flow in this simple network. This line should be as low and flat as possible; low values indicate low delay and a flat slope corresponds with a fluid network (i.e. the delays are well distributed). Opportunities to improve network flow exist where areas of low delay are adjacent to areas of high delay. Ideally, the line should have a value of zero from one end of the network to the other, but it is prohibitively expensive to eliminate all delay from a rail network. In incremental improvement situations, it is generally desirable to let trains advance and incur a little delay at a few locations rather than a lot of delay at one location AREMA

5 BOTTLENECKS, DELAY ACCUMULATION, AND TRAFFIC FLOW A bottleneck is a restriction of flow. Clearly, the bottleneck in this simple network is the long stretch of single track that must be shared by all trains, and this constraint results in delay accumulating at both yards. To address the bottleneck, assume Middle Siding is now added at the network s midpoint. Through simulation, the total system delay is reduced by 55 percent to 72 min / 100 TM. In Figure 3, a new delay zone graph confirms that the delay at the both yards has in fact been reduced, but these reductions are partially offset by new delay at Middle Siding. In this sense, the siding is not just a place to pass trains; it is the strategic tolerance of delay at one location in order to reduce overall delay of the system. Delay now accumulates at three locations in the sample network, and each delay is influenced by the other two. In this case, delay at Middle Siding was allowed to increase from zero to 23 hours per week in order to reduce the delay at both yards AREMA

Figure 3: Delay Zones for Sample Network with Middle Siding Also note that with Middle Siding in service, the traffic flow line is lower and flatter than before.

6 Figure 3: Delay Zones for Sample Network with Middle Siding Also note that with Middle Siding in service, the traffic flow line is lower and flatter than before. Lower corresponds with the lower delay, and the flatter slope indicates the trains are advancing instead of being held at the yards. The network is more fluid. This example also illustrates why capital is sometimes spent at the wrong locations. A bottleneck will restrict traffic flow at one location, but congestion and delay often manifest themselves many miles away. Prior to building Middle Siding, one can imagine yardmasters at both ends complaining about congestion and trying to justify additional yard capacity. But the delay zone graph provides a system-wide view of delay, and in this case confirms that the solution was thirty miles away from each of them AREMA

Taking this one step further, if Midwest Siding is added between West Yard and Middle Siding as indicated in Figure 4, the simulated delay is reduced an additional 32 percent to 49 min / 100 TM.

7 Taking this one step further, if Midwest Siding is added between West Yard and Middle Siding as indicated in Figure 4, the simulated delay is reduced an additional 32 percent to 49 min / 100 TM. As expected, the delay zone graph shows delay now accumulating at four locations instead of three and the flatter traffic flow line indicates increased network fluidity. Figure 4: Delay Zones for Sample Network with Middle and Midwest Sidings INTRODUCING REAL-WORLD PARAMETERS The delay zone graphs in this simple example illustrate a few basic concepts, but they do little more than validate our intuition; we expect that a siding will reduce the delay on a single track network. In reality, rail networks are much more complex and the dynamics of rail network flow may not be as straightforward. Rail capacity is affected by the method of operation, grades, speed limits, dispatching priorities, and many other physical and operational constraints. Unlike 2011 AREMA

To demonstrate what just one variable can have on a network, assume there is now a two percent ascending grade from East Yard to Middle Siding in the previous example.

8 the simple example above, it may not be clear how many bottlenecks exist, where they are located, or where the delays are accumulating. In addition, it may not be clear what impact an improvement has on traffic flow. Delay zone graphs can help provide the necessary insight. To demonstrate what just one variable can have on a network, assume there is now a two percent ascending grade from East Yard to Middle Siding in the previous example. As indicated in Figure 5, the simulated delay for this modified network is now 130 min / 100 TM (as opposed to 72 min / 100 TM in the flat example). The higher delay is the result of trains unable to make track speed going up the steep grade and the resulting longer wait times between train meets. Figure 5: Delay Zones for Modified Sample Network with Middle Siding 2011 AREMA

9 Given past experience, it is logical to expect that delay can be significantly reduced by adding Midwest Siding to the network. However, further simulation indicates that the siding only reduces delay by five percent to 123 min / 100 TM in this modified network. Delay is reduced nonetheless, and a modeler that does not know where the delays are accumulating or understand the dynamics of network flow may recommend Midwest Siding as a worthy capacity investment. But is this the best location? Figure 6 indicates that the introduction of Midwest Siding reduces delay at West Yard and Middle Siding as expected. However, delay actually increases at East Yard. The improvement adds delay to the most congested part of the network. A simulation model is just a tool and cannot predict all outcomes, but if East Yard is already running at or near full capacity, the Midwest Siding project may increase the chances that the yard, and hence the entire service plan, will fail AREMA

Figure 6: Delay Zones for Modified Sample Network with Middle and Midwest Sidings A primary benefit of simulation models is that competing scenarios can be tested to see if there is a better solution.

10 Figure 6: Delay Zones for Modified Sample Network with Middle and Midwest Sidings A primary benefit of simulation models is that competing scenarios can be tested to see if there is a better solution. If Mideast Siding (midway between Middle Siding and East Yard) is added to the model instead of Midwest Siding, the simulated delay is reduced by 54 percent to 60 min / 100 TM. Figure 7 confirms that Mideast Siding reduces delay at all locations, and more importantly, the location with the highest delay sees the most improvement. The traffic flow line confirms that trains are advancing as the remaining delays are more evenly distributed across the network. All else being equal, Mideast Siding is clearly a much better choice than Midwest Siding AREMA

Figure 7: Delay Zones for Modified Sample Network with Middle and Mideast Sidings A USEFUL ANALYTIC TOOL The preceding example illustrates the impact that just one parameter can have on delay in a

11 Figure 7: Delay Zones for Modified Sample Network with Middle and Mideast Sidings A USEFUL ANALYTIC TOOL The preceding example illustrates the impact that just one parameter can have on delay in a simple rail network. Because real networks are dynamic systems influenced by many factors, a delay zone graph is often necessary to understand a network s unique personality. Since 2003, this tool has been a vital part of Norfolk Southern s analytic approach to capacity planning. In that time, over fifty unique networks have been modeled and as a result, NS has invested over $150 million in capacity-enhancing projects that are now in service across the NS system. Additional approved projects are in various stages of design and construction AREMA

Figure 8 is a delay zone graph from a recent NS project. The rail network contains approximately one hundred route miles and is a mix of double track and single track with sidings.

12 Figure 8 is a delay zone graph from a recent NS project. The rail network contains approximately one hundred route miles and is a mix of double track and single track with sidings. Traffic on this line is primarily coal (both empties and loads) and operations are heavily influenced by grades, train length, yard capacity, and operating practices. The method of operation is split between centralized traffic control (CTC) and non-signaled territory. The project objective was to determine the amount of infrastructure required for various levels of projected future traffic. Figure 8: Base and Future Delay Zones for Recent NS Project As indicated, the network s baseline delay is 191 min / 100 TM and the red bars represent the accumulated delay in zones A through P. Zone J is currently the most congested part of 2011 AREMA

13 the network. When future traffic is added to the model, system delay increases 30 percent (to 249 min / 100 TM). However, the green bars illustrate that the additional traffic does not affect all areas equally. While most areas see some moderate increase, delay triples at Zone A and quadruples at Zone C. Furthermore, Zone A now surpasses zone J as the most congested. Through field observation and research, the modeler may have some indication that zone J is currently congested and may design an improvement scenario to address that problem. When one such improvement scenario is modeled, the total network delay reduces 33 percent to 166 min / 100 TM. This is 13 percent lower than the baseline delay and the improved network with additional trains seems to run better than the original network with base level traffic. A modeler that relies on this single metric seems to have data to support a recommendation for these improvements. But Figure 9 provides a different view. As indicated by the orange bars, the improvement scenario reduces delay at Zone J as designed, but delay increases even further at Zone A. The dynamic nature of network flow is demonstrated again, and in this case, the improvement in one area has come at the expense of another. The extreme slope of the orange traffic flow line further illustrates that traffic is not fluid between Zones A and E AREMA

Figure 9: Initial Improvement and Final Improvement Delay Zones for Recent NS Project In this real-life example, the delay at Zone A represents loaded coal trains being held out of a yard due to a

14 Figure 9: Initial Improvement and Final Improvement Delay Zones for Recent NS Project In this real-life example, the delay at Zone A represents loaded coal trains being held out of a yard due to a lack of yard capacity. The proposed future traffic turns the yard into the new system bottleneck, and a modeler that recommends this improvement scenario based on the 33 percent reduction in normalized network delay is setting up the yard for failure. With the knowledge gained by the delay zone graph, an alternate improvement scenario that includes two additional yard tracks may be modeled. As indicated by the blue bars in Figure 9, this alternate scenario allows for further reduction in total network delay, but equally important, all major bottlenecks have been addressed and the network flows more fluidly AREMA

15 PUTTING DELAY ZONES INTO PRACTICE Delay zone graphs can only be created if the simulation software can measure and report delay within user-defined zones. With the proper software in use, the first step is to determine and define the delay zones. It is up to the modeler to choose the appropriate level of granularity to fit study objectives and network capabilities, but generally speaking, each zone should define an area for which delay measurement may be relevant. For example, it may be worthwhile to know how much delay occurs at a specific siding due to train meets. To capture this delay, the entire siding and the portion of main track between opposing switch points would be assigned to a unique zone. If desired, the modeler can choose to differentiate the siding from the main track or eastbound delays from westbound delays by assigning smaller zones. Other zones may be defined to include entire interlockings, yards, the stretch of track between adjacent interlocking, etc. While delay may not accumulate in every defined zone, it is important that the zones be comprehensive and contiguous across a rail network. One possible selection of delay zones is illustrated in Figure 10. After a simulation has completed, a delay zone graph may be prepared by plotting the accumulated delay for each zone in sequential order AREMA

Figure 10: Possible Selection of Delay Zones The delay being measured should include all unplanned dwell (for train meets, waiting for schedule, handling track hardware, etc.).

16 Figure 10: Possible Selection of Delay Zones The delay being measured should include all unplanned dwell (for train meets, waiting for schedule, handling track hardware, etc.). Delay also includes the corresponding deceleration and acceleration; however, this can be difficult to accurately assign because a train that stops for a meet in one zone may have decelerated in the previous zone. Experience has shown that the losses due to deceleration and acceleration may be ignored, and the examples presented herein use stop delay as a proxy for total delay. CONCLUSION There are many factors involved in deciding when and where to invest in new rail capacity. Obviously, capital constraints must be met and all improvements should be aligned with the company s strategic goals and objectives. Within those guidelines, many railroads today use simulation models to help identify and prioritize new capacity investments AREMA

17 Modelers typically simulate various improvement scenarios and measure how well each reduces a network s delay per 100 train miles. This provides a normalized view of network delay and facilitates comparisons between competing scenarios, but as demonstrated in this paper, delay is not a homogenous value that applies evenly across a network. Delay accumulates at specific locations in response to one or more bottlenecks. Often, these bottlenecks are many miles away from where the delay occurs. A rail network is a dynamic system, and the specific delays that accumulate at the various locations are related to one another. If delay is allowed to increase or decrease at one location, the delay at all other locations in a network will be affected. As demonstrated, it is possible to reduce total system train delay while actually adding more delay to an already congested part of a network. To prevent this, a modeler needs to know where delay is accumulating and understand the relationship between bottlenecks, delay accumulation and traffic flow. The delay zone concept presented in this paper will allow a modeler to see the delays and understand how a network responds to various improvement scenarios. With this knowledge, the modeler and will be able to make more effective capacity investment decisions. LIST OF FIGURES Figure 1: Sample Network Layout Figure 2: Delay Zones for Sample Network Figure 3: Delay Zones for Sample Network with Middle Siding Figure 4: Delay Zones for Sample Network with Middle and Midwest Sidings Figure 5: Delay Zones for Modified Sample Network with Middle Siding Figure 6: Delay Zones for Modified Sample Network with Middle and Midwest Sidings 2011 AREMA

18 Figure 7: Delay Zones for Modified Sample Network with Middle and Mideast Sidings Figure 8: Base and Future Delay Zones for Recent NS Project Figure 9: Initial Improvement and Final Improvement Delay Zones for Recent NS Project Figure 10: Possible Selection of Delay Zones 2011 AREMA

19 Michael K. Williams, PE, MBA Manager Industrial Engineering Norfolk Southern Corporation

20 Rail simulation modeling and metrics Limitations of most common metric Demonstration of alternate method Simple model Include real-world parameter Industry example Improve understanding of network dynamics How to apply new method

21 Commonly used in rail industry Evaluate track capacity Quantify the impact of: Infrastructure Operating plans and procedures Typical metrics: Train delay (delay per 100 train miles) - most common Train speed Fuel consumption On-time performance Others

22 Good High level view of network health Normalized value; easy to understand Allows for easy comparison of scenarios Bad Does not tell the whole story Masks network dynamics Ugly Making decisions based on this single metric can waste money

23 Where are the bottlenecks? Where does delay accumulate? Do trains flow or hurry up and wait? How do these factors change in response to an improvement scenario?

24 Eastbound Trains Westbound Trains West Yard System Delay = 159 min / 100 TM East Yard

25 Hours of delay per week West Yard System Delay = 159 min / 100 TM The single track is the bottleneck, but delay accumulates at the yards East Yard 0 West Yard East Yard

26 West Yard Middle Siding East Yard 120 Base Delay = 159 min / 100 TM Delay With Middle Siding = 72 min / 100 TM 55% Improvement Hours of delay per week Congestion occurs at the yards, but the solution is in the middle of the network 0 West Yard Middle Siding East Yard

West Yard Midwest Siding Middle Siding East Yard

Sidings = 49 min / 100 TM Additional 32%

27 West Yard Midwest Siding Middle Siding East Yard Hours of delay per week Base Delay = 159 min / 100 TM Delay With Middle Siding = 72 min / 100 TM Delay With Middle + Midwest Sidings = 49 min / 100 TM Additional 32% Improvement 20 0 West Yard Midwest Siding Middle Siding East Yard

28 Simple example only validates intuition Capacity is affected by many factors: Method of operation Grades Speed limits Dispatching priorities Many others

29 2% Ascending Grade West Yard Middle Siding Delay With Middle Siding = 130 min / 100 TM East Yard Hours of delay per week West Yard Middle Siding East Yard

30 2% Ascending Grade Hours of delay per week West Yard Midwest Siding Delay With Middle Siding = 130 min / 100 TM Delay With Middle + Midwest Sidings = 123 min / 100 TM Middle Siding 5% Improvement (Compare to Prior 32%) Overall delay reduction, but more congestion at East Yard East Yard West Yard Midwest Siding Middle Siding East Yard

31 2% Ascending Grade West Yard Middle Siding Mideast Siding East Yard Hours of delay per week Delay With Middle Siding = 130 min / 100 TM Delay With Middle + Mideast Sidings = 60 min / 100 TM 54% Improvement Delay reduced at ALL locations 10 0 West Yard Middle Siding Mideast Siding East Yard

length of single track, and operating practices Goal: determine amount

32 100 route miles Heavy tonnage coal route Mix of single track and double track Mix of CTC and non-signaled Influenced by yard capacity, grades, length of single track, and operating practices Goal: determine amount of infrastructure required for various levels of projected future traffic

33 60 50 Baseline Delay = 191 min / 100 TM Hours of delay per week In baseline condition, Zone J is very congested 0 A B C D E F G H I J K L M N O P Delay Zones

34 Hours of delay per week Baseline Delay = 191 min / 100 TM Future Delay = 249 min / 100 TM Future traffic causes overall delay to increase 30%, but not all zones affected the same: + 25% at Zone J + 200% at Zone A + 300% at Zone C Bottleneck has shifted 0 A B C D E F G H I J K L M N O P Delay Zones

35 80 70 Baseline Delay = 191 min / 100 TM Future Delay = 249 min / 100 TM Hours of delay per week Future Delay With Improvements = 166 min / 100 TM Improvements reduce overall delay by 33% (to 13% lower than base), but Zone A is in a meltdown situation. Additional improvements needed. 0 A B C D E F G H I J K L M N O P Delay Zones

36 80 Baseline Delay = 191 min / 100 TM Hours of delay per week Future Delay = 249 min / 100 TM Future Delay With Initial Improvements = 166 min / 100 TM Future Delay With Final Improvements = 106 min / 100 TM Additional improvements address the new bottleneck and reduce delay at all locations. Traffic is much more fluid A B C D E F G H I J K L M N O P Delay Zones

37 Software must be able to measure delay within user-defined zones Define delay zones in model; Consider: Study objectives Model capabilities Level of granularity required Zones should be comprehensive and contiguous Plot the delays in sequential order

38 Zone 1 (yard) Zone 3 (double track) Zone 4 (single track) Zone 2 (double track in front of yard) Zone 7 (double track) Zone 6 (single track) Zone 5 (siding)

train delay and understand network dynamics the

39 Simulated delay values are useful, but they do not tell the whole story Delay zone graphs allow a modeler to see train delay and understand network dynamics the relationship between: Bottlenecks Delay accumulation Network flow

miles away Rail networks are dynamic It is possible to reduce

40 Delay is not homogenous; it occurs at specific locations Bottlenecks restrict flow, but delay often materializes many miles away Rail networks are dynamic It is possible to reduce overall network delay while increasing the chance for system failure

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