By P Martin

Halcrow Group Ltp, Great Britain

 

Introduction

Heavy and Light rail systems the world over are expanding to meet new demand for additional capacity wilest facing with even tighter controls of budget and funding.

 

In this environment it becomes more and more vital that a project can demonstrate that it works and will deliver the desired be befits and, where options exist, that the best one is selected.

Simulation and scheduling software can take a major role in informing the decision in a project. This paper aims to outline the type and application of simulation and scheduling software.

 

Simulation systems come in a variety of forms from specialist tool aimed at a specific question to general purpose systems that are ca[able of addressing a number of railway scenarios.

 

Single Train Simulation: These simulators consider the actions of one train running over a declared infrastructure.  They will model the actions of a single train in great detail but they do not consider the interaction of that train with other trains in a network.

Multi-train Simulation: As the name suggest a multi-train simulator will consider the interaction between many trains in a network operating to a timetable and interacting with the track layout and signaling systems.

 

Scheduling Systems: Such system are used to plan and develop timetables.  These are not strictly simulation systems in that the trains don’t move in anyway but they are used in close association the simulation tools which provide the inputs and can be used to evaluate the output.

 

Power Supply Simulation: Once you know how many electric train you are to going run and how much power each will require a power network simulator can take power demands from your train simulator and use them to model the electrical supply network.

 

Integrated Simulation: The intergraded simulation & scheduling tool is the step in the development of these tools.  The scheduling systems can create a theoretical timetable and the simulation tool can try it and see if it works based on actual train performance.  It therefore makes sense to combine the two and test timetables in the same tool.

 

SINGLE TRAIN SIMULATION

 

Other papers have covered principals of simulation and the physics involved in the equations of motion. Given that a simulator must apply the laws of physics to the calculation of train movement al the commercial simulators must be very similar in the calculation they perform.

 

The key difference is therefore in the user interface, case of use and the presentation of the simulation results.  Data entry can vary from a spreadsheet in which cells are completed by the user to fully interactive user interfaces.

 Speed of use of simulation tools can be aided by the use of typical values used unless a specific value entered.  This can produce a quick answer that can be refined and improved as more data becomes available.  Another approach for existing railways is the development of infrastructure databases that can be loaded directly to simulators. 

 

 

Basic Simulation

 

In the basic simulation a train accelerates due to the installed power applying a force greater than its resistance. The accelerating force decreases and the resisting force increases as speed increases until a balance is reached. This is the balancing speed and this will be maintained on straight and level track until the accelerating force is removed and replaced by a braking force to overcome the trains’ momentum and stops the train.

 

Figure 1 shows a speed distance plot for a train accelerating until a balance is reached. The tractive effort and resistance curves show that balance isn’t reached until a point beyond the service maximum speed.  The tractive effort and resistance curves are shown in figure 2.  An exaggerated resistance curve shows how a balancing speed lower than line speed would prevent a train reaching the line speed.

 

The data in the example is for a Docklands Light Railway B92 train running a line voltage of 675 volts and loaded to a crush laden condition.

 

Speed

 

A train’s performance will be designed so that the balancing speed is above the normal operating maximum speed plus some allowance for maintaining performance on gradients. The degree of performance on gradients will be influenced by  service requirements and costs.

 

Speed of trains within a simulation may be controlled by a number of parameters. For Example

• Line Speed

• Trains Maximum Speed

• Conditional speeds {temp speed restrictions, turnout speeds. Ect}

The active speed limitation at any instant will be the lowest applicable at the location. On all railways most speed restrictions apply to the full length of the train.  However this applied can vary.

 

On heavy rail lines the restrictions end point is marked and it is left to the driver to work out when the rear of his train is clear of the restrictions whilst on other lines with fixed formation trains such as trams and metros the end is marked at the point the driver cab accelerate.

 

For any given line there can be a number of speed profiles applied. These typically allow different speeds for lightweight trains, tilting trains or heavy freight trains.  In addition to the speed profile data there will be information on gradients and curvature for a route which will affect the speed that can be achieved. 

 

Figures 3 & 4 show speed plots for trains over a route with and without gradients.

 

Time

 

A single train simulation can determine the time will take to run between stations and can be used to compare the performance of a number of different train types.

 

At a simple level a train with a higher maximum speed should complete a run in a shorter time than one with a lower speed. In practice though a train with a lower top speed but better acceleration may perform better over a given infrastructure.

 

Times generated by single train simulation also form the basis of the input into timetabling systems.  By running a series of simulations all the running time options between pairs of stations can be generated.

 

The table below shows an example for a pair of stations 16.5km apart.

 

A set of similar run times for each type of train between each pair of stations on route from the basic building block for a building a timetable. These run time are often referred to as standard run times or SRT’s.

Energy

 

Single train simulation will provide a calculation of the energy required for the train to run. Typically this will be a figure for the mechanical energy in kilowatt hours and can be used as an early input into the design of an electrification scheme by multiplying the energy figure by the number of trains to be run.  This of course makes no allowance for the performance of the electrification distribution scheme but is an early guide.

 

For a diesel train the fuel consumption of the train can be calculated given data on consumption rates of diesel engines.

 

In the examples considered differing energy requirements can be seen and are summarized in the table below.  Additionally the effects on time and energy are shown for two further scenarios.  In the first, the 40 km/h speed restriction is removed and, in the second the train performance is eased at a cost to increased journey time.

 

The initial run over the flat railway gives base energy and run time figures, the second run with the gradient demonstrates the impact of the gradient on run time adding two minutes to the journey and increasing the power consumption by 62%.

 

The additional runs shown in table 2 demonstrates the impact of removing the short 40 km/h speed restriction, if this were possible, and not running the train at its full performance.  Still using full performance more than a minute can be saved or by easing the performance so the journey time about the same 9.6 KW/h can be saved.  This might not sound like much savings but it will mount up over time, a typical six train per hour and fourteen hours per day it amounts close to 300 MW/h per year.

Another option to consider is to extend the journey time allowing the train to coast for a proportion of the journey. This can demonstrate a savings in energy savings for a modest extension on run time can produce energy savings of up to 20% on a surban system depending on service pattern and frequency.  A further benefit of this scenario is that if the use if coasting is relate to the schedule a late running train may not coast and recover from running late.

 

Timetable

 

  Run times for single trains form the basis of the production of timetables. They are often referred to as SRT and often they have historical reasons for not being purely the running time.

 

Timetables were originally produced manually on traingraphs.   This process relied on the experience of the timetable compilers to judge what was practicable and for the resolution of conflicts.  This process meant that, by and large, the timetable developed slowly with each successive timetable issue featuring only small changes and incorporating lessons learned through practical operation of the last timetable.

This process was ripe for improvement by the use of computers and the first generation of scheduling systems have been in use for some years. These can replicate the graph drawing process in a much shorter timescale and cope with larger changes in a single iteration. A drawback of these first generation systems was the limit on the size of network they could handle. With a system the size of the UK rail network the scheduling exercise had to be broken up in to zones. This created problems, typically, that a train crossing many zonal boundaries cannot be viewed in its entirety. The result of this is that the impact of changes proposed by one zone cannot be viewed in other zones.

First generation scheduling systems made the best use of the computing power available at the time. The latest generation of scheduling tools exploit the advances in both computing power and network connectivity. Current systems in use with Network Rail work from a single database and are available right across the Network Rail communication network. It does all the first generation system could do and more.

It can now handle the whole of the UK trains services in one database. This allows complete cross-zone trains to be viewed whilst access controls limit authority to alter these trains.  Currently the system is used for the regular timetable changes but it is being developed to handle short and very short term planning requirements.

The system supports the bidding process under which train paths are now allocated on the privatised railway as well as short term planning. The system produces ready to use train graphs and print ready timetables and is being developed to fulfil a number of other business aspirations.

Despite all the added functionality the basis for calculation remains the run time between stations for each train type. These run times are still generated by performance simulation and simulation is being used to challenge some of the historical SRT’s.

Figures 5 & 6 show TrainPlan screens for trains between Leeds and Scarborough in table and train graph form.

 

Project Development

Matching Train with Infrastructure for

Cost Effective Improvement

The railway as a whole is made up of many facets that interact to produce the whole railway. It should be obvious that the design of each element must consider the others.

In a very extreme example it is obvious that the track design team and the rolling stock designers should agree a fundamental such as gauge at a very early stage.

The development of a project will start with an iterative process considering each of the topics detailed below. After the first simulations the process is refined and simulated to develop the optimum solution. Finally this process leads on to multi train simulations to verify that all the aspects of design will work together to support the planned train service.

 

Civil Design

 

The civil design for a new project or an upgrading project starts from broadly the same point. For a new project, there would be some idea of the proposed alignment whereas for an upgrading project this would be the existing infrastructure.

Initial simulation runs will calculate the theoretical maximum speeds derived from the alignment data and by applying rules for cant and permissible cant deficiency. Additional constraints can be factored in at this stage such as speed restrictions applied for tunnels as a result of air pressure limitations.

This will often produce a very castellated speed profile that the trains may find difficult to follow requiring repeated acceleration and braking reducing passenger comfort and increasing energy costs. Further simulations will refine this combining it with other considerations detailed below.

Providing and maintaining infrastructure for trains to operate on is an expensive business. As a generalization, providing infrastructure for trains to run at high-speed costs more than it does to run at low speed. It therefore follows that it is sensible to match civil line speed with the speed capability of the trains using the lines.

Historically the railway infrastructure was built in at a time when present day speeds weren’t even imagined. This provides a constant challenge to the designers trying to get every last scrap of speed out of the infrastructure.

 

Signaling Design

The signaling system is required to be able to support the aspirations for both speed and capacity. The governing factor in signaling design is the ability of the train to stop i.e. the braking distance. 

Speed

The speed at which a train can run is determined by the distance it takes to stop so that from receiving the first caution signal it can be brought to a stand before the stop signal. In designing the signal spacing a degraded brake performance is until listed to provide a safety margin Thus a better braking performance will allow faster speeds for any given signal spacing.

Where signaling already exists faster speeds have been achieved by improvements in brake performance. Examples of this are the HST which can stop from 125 mph on lines originally signaled for 100 mph and the newer tilting trains designed to match this performance from 125 mph even if they are capable of higher speeds with new signaling.

Thus for many passenger trains on the Network Rail system there is the possibility of two speed profiles, one for modern trains built with 9%g braking capability, and a lower one for older trains with a 7%g braking capability. Other types of trains can have yet further speed profiles.

 

Headway

Headway is the separation between successive trains. This is usually expressed as a time in whole minutes and is based on the following train having clear signals. In its simplest form this is expressed for similar trains running at the design speed. Once again this is governed by the signal spacing. Whereas for speed greater signal spacing allows higher speeds this increases the headway. To give a close headway signals need to be closely spaced.

Figure 7 demonstrates the signaling speed and headway’s. Clearly a compromise is required even at this initial level.

 

Power Supplies

 

Electrical engineers have to consider the supply network for an electrified railway. A summary of the considerations follow.

For a new railway the initial design will be based on the theoretical power consumption of a single train multiplied by the number of train paths. From this theoretical demand constraints will be added to refine the design. Those constraints will be such factors as the location of the National Grid feeders.

On an upgrading project the constraint will be the existing supply and its strength.

The initial studies can only be a guide as the capacity and a simulation of the full service only can test resilience of the supply. This will mimic the capacity of the supply with the dynamic interaction of all the trains with the proposed train service.

Factors to be considered are:

Substation Sizing:

The initial calculations will determine the required capacity of feeder stations and consider such constraints as any limitation in the capacity of the supply from the National Grid. It is almost inevitable that the supply points will not coincide with the optimum designed location.

Voltage Drop:

Voltage drop in the supply for the proposed loading will govern the ideal feeder station placing.

Once the actual feeder station locations have been determined the voltage drop calculations will be revisited to ensure that the supply remains robust.

If not already dictated the voltage drop calculations may influence the voltage and distribution system chosen for the railway.

 

Energy costs:

Again if not already dictated the cost of energy distribution may influence the choice of supply. Long distance railways favor higher voltage systems to minimize the investment in equipment and to reduce the effects of voltage drop.

As energy is usually paid for on the basis of an amount per unit with a multiplier set by the maximum demand level, and a peak free supply is beneficial although the majority of this is determined by the proposed train service.

Regeneration:

The use of regenerative braking can have a major impact on energy costs. It can also have benefits for the rolling stock engineer in reducing the frequency with which friction brake components require attention.

The system design will have to incorporate extra equipment if regeneration is planned however the supply capacity should be designed assuming that regeneration does not occur.

Provision of regeneration thus becomes a balance between the projected energy saving and the cost of providing additional trackside and train equipment. The benefits to be gained from regenerative braking will depend on the type of train service planned. A frequent commuter stopping service will give a higher return than a fast non-stop service. Simulation provides the basic data to undertake such evaluation.

 

Operations/Capacity

Speed and Headway have been mentioned under signalling from the point of view that the design of the signalling controls the absolute values for these. They require further consideration in relation to the business aspirations: -

 

Speed:

As demonstrated earlier the route maximum speed is determined by the signalling design and mitigated by other engineering and energy considerations. Operationally it is likely that not all trains will run at this maximum speed. Business targets, with a mix of fast express passenger service, lower speed passenger services and slower freight trains may ultimately influence the speed.S