Paweł Kołodziejczyk, M.Sc.

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Studying bottlenecks with simulation models

In the railway domain the goal of many stakeholders is to have an optimal utilisation of the rail network. In order to achieve optimal utilization for the rail network, specialized simulation software is used nowadays. One of the great things about current simulation models is that they reveal bottlenecks. As we all know, bottlenecks impose delays and restrictions in the normal flow of transportation. Even though they occur in a limited area, they have a negative effect on the entire system. There are 3 major types of bottlenecks: infrastructural bottlenecks, regulatory bottlenecks and operational bottlenecks. In this blog we discuss the emergence of bottlenecks, their effect on capacity and the overall influence on rail networks.  

The different types of bottlenecks

As we stated in the introduction, there are 3 major types of bottlenecks; infrastructural bottlenecks, regulatory bottlenecks and operational bottlenecks. Infrastructural bottlenecks can be caused due to chronical or temporary conditions. Chronical conditions are for example physical restrictions that form bottlenecks as traffic expands. Temporary conditions can be caused by natural or market forces.

Regulatory bottlenecks are delays caused by regulations. Good examples of regulatory bottlenecks are for example inspections by the customs. These inspections cause a delay in the total transportation system.

Operational bottlenecks are caused by specific tasks and procedures in the management of transportation modes and terminals that trigger bottlenecks. From a capacity perspective, the availability of equipment and vehicles can create bottlenecks as the necessary conveyances may not be where their capacity is needed.

studying bottlenecks with simulation

What are bottlenecks in the railway industry

Bottlenecks cause delays and these delays affect the capacity of the entire system. A delay is not a homogeneous value that applies equally across a rail network; delays accumulate at specific locations. 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.

Using simulation is a reliable approach to allow modelers to better determine where network delays are accumulating and understand the relationship between bottlenecks, delay accumulation, and rail network flow. By analysing a network’s “delay zones”, an analyst is able to pinpoint the delays, determine their magnitude, and measure how well each respond to new improvement scenarios. With this knowledge, the modeler is be able to make more informed capacity investment decisions.

Are bottlenecks always infrastructural?

It is a common concept in the industry that a bottleneck is always due to insufficient capacity on a small part of the infrastructure. There are plenty of arbitrary statements such as “This bridge is an infrastructural bottleneck” or “If only not for this tunnel we would be able to increase the number of train connections”. In fact, the problems are rarely as simple as that and are connected to how the infrastructure is used, e.g. are there any unnecessary moves, is there wasted time, inefficient scheduling, can the trains be longer etc. Capacity is always a combination of factors, and these are not limited to infrastructure, rolling stock or operating strategies .

A lot of factors, often in combination, will influence capacity. Train lengths and used locomotives, especially in terms of acceleration and maximum speed are prime examples. For infrastructure we mostly distinguish number of tracks, switches and the signalling and protection system. Operating strategies might involve number and durations of stops trains make, number of moves of separate locomotives, their routing or even number of trains allowed in a certain area.

The effects of bottlenecks on capacity

In the past capacity studies were done in a reactive manner based on signals coming out of the operational management of the port railway infrastructure, which was due to historical reasons, e.g. “we’ve always been doing it this way” and lack of tools able to cater more advanced demands. From now on out, the aim is to take proactive steering approach that takes into consideration tactical/strategic concern on capacity management. This entails forecasting future scenarios for railroad freight transport, so that problems can be solved using structured and repeatable processes in time. Tooling is important in realising that desired capacity management methodology. The tooling should be able to address major capacity elements such as those addressed in the table below wherein a small comparison between Excel and simulation tools are made.

How to identify bottlenecks

In some cases, it might not be that difficult when the information is known from operational experience. It happens quite often that operational difficulties are the reasons studies are conducted. It is thus not necessary to re-discover them, yet analysis in a model should at least provide a quantitative scale of the problem and help discover its true origin. It can be surprising how often there is knowledge about a problem and despite that there is little understanding towards how much of an issue it really is and most importantly why.

It is more challenging to identify bottlenecks that could happen and the specific conditions necessary to cause them. Yet in many cases pinpointing a possible problem is a necessary step for avoidance. Whenever a change or intervention to a system is introduced, there is a risk of creating one or multiple bottlenecks. If there aren’t any, a quick check of most important KPIs will prove that. If the turnaround times are consistent, delays as well as occupation manageable, there are probably no bottlenecks. If one or more of these KPIs increases, especially when sharply in time, then a bottleneck is likely.

Pinpointing a specific location and its start might be difficult as potentially there might be a lot to investigate and there is no single rule on how to approach it. What is helpful is a capable analyst experienced with the software that is enough for the purpose and has detailed visualisations. A few tips that generally work is to start from identifying specific locations and trains affected when delays started. Then use animation to investigate the interdependencies among trains in the system. Find what started that and move forward in time, checking out other necessary KPIs to fully understand the issue.

How to solve bottlenecks

Once a bottleneck is identified and assessed, steps can be taken to solve it. As previously mentioned, it usually is a combination of factors including, but not limited to infrastructure, rolling stock and operating procedures.  The solution thus, might include one or multiple steps, and will be the spot where different stakeholders will disagree on the best course of action. Infrastructural solutions are the simplest to come up with, and most likely the most expensive to execute. On the other hand, better coordination and operating strategies are challenging to agree among stakeholders.

There are, however, a few tips to provide on simpler measures that can be taken, which depending on the actual situation might be for example:

  • Limit the number of unnecessary moves, by e.g. separate locomotives
  • Reduce idle time on sidings
  • Consider stopping trains earlier before areas become overfull and experience snowball effects for delays
  • Prioritise departing trains
  • Consider increasing the length of trains
  • Be flexible in assigning resources to maximise throughput

The final one, which is not trivial, is to improve planning and scheduling to avoid peaks in arrivals and multiple trains trying to enter and claim the same area at the same time. Evenly spread arrivals and taken routes are of course difficult to execute, especially for freight trains.

Decision power is important in capcity management

Conducted capacity studies sometimes lead to results that have a call to action. These studies then have a clear conclusion about e.g. capacity bottlenecks, potential improvement areas or other opportunities and threats that need resolvement. In these cases, it is desirable that a clear ‘chain of command’ is visible, since the capacity studies success falls or stands with the follow-up after it. Therefore, organisations working together should organise escalation options within their hierarchy to be able to respond adequate to capacity interventions and prevent endless researching/writing from delaying intervention.

An important moment where decision power and clear process agreements are necessary is when deciding upon the logistic parameters, i.e. the assumption necessary for running the simulation. A good way of ensuring the traceability and reliability of estimations is through expert sessions. Certainly, if parties commit to the composition of the expert panel and their judgement upfront, this can ensure higher quality of the capacity study.

|Railway companies are nowadays facing a lot of challenges. The main challenge for them is to optimize rail infrastructure, maximize rail utilization and satisfy freight operators. Solutions to these challenges are restricted by safety regulations and the desire to be energy efficient.


Read our blog about longer trains.

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