Track layout guide

This guide is currently under construction!

This guide is intended to develop player knowledge relevant to building efficient track layouts in NIMBY rails.

It consists of two parts: In the first it explains fundamental concepts necessary to create efficient track layouts, and in the second it shows a number of examples of challenges that are regularly encountered in a network and demonstrates how track layouts can be used to solve these.

Part 1: Fundamental principles for efficient track layouts

This part covers a number of fundamental principles the player can use in analysing the performance of track layouts. It gives an overview of what is meant by efficiency with respect to track layouts, the principle of train paths and how this is used in understanding how trains use a given track layout, and how scheduling, track layouts, and service patterns all interact together in a network.

Efficiency of track layouts

What is meant by efficiency of a given track layout?

Three main metrics can be used:

• Capacity, that is to mean how many trains can use a given section of track in a unit of time (usually measured in trains per hour or paths per hour).
• Speed, the speed that trains can pass through a section of track, most importantly how it affects the average speed between stops and over the whole length of the line.
• Complexity of the track layout itself, which can encompass costs of construction, space requirements, and even the ease of scheduling services onto the track.

Capacity of track

The analysis of the capacity of a section of track can range from very simple to very complex.

Basic calculation of track capacity

In the most basic form, the capacity of a section of open track can be calculated by taking the interval by which trains can pass this section of track. For example, if a section of track is able to have a train pass every 5 minutes, one after the other, it would be able to carry 12 trains every hour. The track therefore has a capacity of 12 tph, tph standing for trains per hour.

The same principle can be applied to a junction: If only one train can occupy a junction at one time, and it takes 3 minutes for a train to pass the junction, this junction can be said to have a capacity of 20 tph.

Trains of different speeds on the same track will use extra capacity

On a section of open track, two trains of the same speed will exit the section of track with the same spacing as they entered it. If one of them is faster than the other, the spacing will change. A fast train travelling between two slow trains will get closer to the slow train in front of it.

The following is a simple example:

Fig 1

We have two stations, A and B, separated by a section of open track. The slow trains, trains 1 and 3, take 60 minutes to travel the distance between the two stations. The fast train, train 2, takes 50 minutes to do the same. The track between the stations allows 1 train every 5 minutes to pass.

To maintain the 5 minute spacing between the trains, train 2 must leave 15 minutes after train 1, this ensures that when train 2 reaches station B, it is there 5 minutes after train 1. Train 3 can start 5 minutes after train 2, but because it is slower than train 2, it arrives at station B 15 minutes after train 2.

In this example, the theoretical capacity of the line if all trains were travelling at the same speed would be 12 tph.

Fig 2

When we expand the table to fit a full hour (train 7 would leave at 7:00, in the next hour), we can see that the differing speeds between the trains trains has eaten up half the capacity, the track can only carry 6 tph.

Track capacity on junctions

A junction may be able to handle two trains simultaneously, this depends on the route the trains take.

As an example, this simple crossover would only be able to take one train at a time in the situation where Train 1 wants to go from Platform A to Track B and Train 2 wants to go from Track A to Platform B. (Fig 3)

However, if Train 1 goes from Platform B to Track B, and Train 2 goes from Track A to Platform A, the two trains can run simultaneously rather than one after the other. (Fig 4)

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  Fig 3

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  Fig 4

Speed

Speed is important because passengers want to get from their departure point to their destination in a timely manner.

The primary way track layouts limit speeds is through curve radius. Switches often have a low curve radius to avoid them taking up a lot of space, so they will often limit the speed of trains using them.

Certain track layouts can allow less switches to be used, this will allow larger switches which can have less tight curve radii. This allows trains to traverse the switches at a higher speed.

Sometimes tight curve radii can not be avoided, in this case, keeping switches and other tight curves close to stations is highly recommended, this allows trains to accelerate to line speeds as soon as possible. Tight radius curves or switches in the middle of a line will cause the train to have to decelerate and accelerate again.

Complexity

As mentioned above, the primary reason complexity of a track layout is important is because of cost. More tracks will cost more to build. The use of bridges and underpasses can help separate trains, however these also have a higher construction cost.

Secondarily, more complex tracks can also be more difficult to manage. A very complex junction might make it difficult for the user to select train paths that do not conflict. Trains can also end up taking unintended paths, and the user might have more difficulty in figuring out what is causing a train to use a different path than intended.

For real life operators, complexity can have serious implications on reliability and maintenance costs. Switches are moving parts, so adding more of them creates more points of failure. The electronics and mechanics can be subject to water damage, they can be jammed by snow or other debris, and they can fail due to routine wear and tear. Most switches also generate a large amount of noise when trains pass over them, this can be bothersome to people in the surroundings. Maintenance costs of switches and structures such as bridges and underpasses also need to be taken into consideration as they will substantially affect the operating costs of the railway. However, as of version 1.6, these effects are not simulated in NIMBY Rails.

Train paths

A train path, broadly speaking, describes the route a train takes and the track it occupies at any specific point in time.

Fig 5

Understanding train paths

Generally a train path can be formed of a route (one or a series of stops and/or waypoints) that follows a specific set of tracks, and a series of times associated with the different locations along the route. To make the path even more specific it can contain speed information too.

Figure 5 shows a simple depiction of a train path as can be found in many route planning apps used by train passengers.

A train path may only contain timing information for large stations along a route. It can also be very detailed and consider timings for every single waypoint along the route, that way allowing very accurate pinpointing of the time the train will pass along the route.

In NIMBY Rails, train paths may be visualised either in the service schedule or in the line editor. The line editor however displays a general train path which must be associated with an order, the order will determine the starting time and the other times will be offset to match the modified starting time.

Path conflict and separation

A path conflict is when two trains will occupy a given section of track at the same time. The converse of a path conflict is known as separation, this entails that trains do not have any type of conflict with each other. Two trains with non-conflicting paths can be said to be separated.

Examples of types of separation

Broadly speaking there are two types of separation, these are time separation and route separation, the following examples offer an explanation.

In the previous example in figures 1 and 2, the trains 1 through 6 all take paths separated by time along the same piece of track. These paths do not conflict because each of the trains passes over a given section of track at a different times. These trains are said to be time separated.

If Train 2 was instead to depart at 6:05, there would be a path conflict, because it would have to overtake Train 1 at some point along the line.

In the example in figure 3 there is a path conflict if we want the trains to use the junction at the same time. Using time separation the path conflict can be avoided, the trains will arrive at the junction at a different time.

In the example in figure 4 there is no path conflict as the paths do not cross, the two trains are therefore route separated.

Examples of path conflicts

On one-way track as is used in most double track routes, there are three main categories of path conflict: Speed conflict, merging conflict, and route conflict. Some examples are given below. Additionally, on a single track route, direction conflicts may be encountered (these will be dealt in a Single-track guide).

Speed conflicts occur when two trains run along the same length of track with a slower train in front of a faster train. As dealt with in the first example of time separation, if in figure 1, train 2 was to depart at 6:05, 5 minutes after train 1, these trains would have a speed conflict. Train 2 would not arrive at Station B at the expected time, 50 minutes after its departure. Such a conflict is regularly encountered when scheduling a combination of intercity and stopping services on the same track.

Possible solutions to a speed conflict may include: Path separating the trains of different speeds by having extra tracks, so quad tracking the line and running slow trains on one pair of tracks, and fast trains on another pair of tracks. Path separating the trains by means of a timed overtake, adding a short passing track for the slow train to stop in while the fast train is allowed to pass in front. Time separating the trains by ensuring that the slow train instead departs after the fast train, eliminating the need to overtake. Time separating the trains by running all trains at the same speed.

Merging conflicts occur when two trains come together from different lines onto one line at the same time. These can be solved by time separating the trains, allowing them to join the shared track one after the other, or building parallel tracks to allow the trains to be path separated.

Crossing conflicts occur when two trains paths on different track intersects at a single point. This was dealt with in the example with figures 3 and 4. There is a crossing conflict when the trains use track a and platform b, and track b and platform a respectively. These are most common in the entrances to large stations or when joining quad track lines. Such crossing conflicts may be solved by means of time separation, scheduling the trains to arrive at the crossing point at different times, or by using overpasses or underpasses to route separate the trains.

The impact of train paths on efficiency

As can be seen, basically all conflicts can be dealt with through time separation, however the reciprocal of this is that where path conflicts are time separated the capacity of the infrastructure is reduced. In order to separate these paths without reducing the capacity, construction (and therefore cost) is required to compensate.

For this reason, when designing a schedule, it is important to create routes and train paths that do not conflict with one another.

Track occupancy

Track occupancy is the time that a train occupies a given section of track.

Part 2: Common track layout challenges and techniques

In this part a number of common scenarios where efficiency of track layouts is important are considered. These are intended to provide an overview of places where track layouts can provide substantial benefit and show a number of techniques to increase their efficiency.

Line and routing considerations

Fundamentally the design of track layouts should follow from the desired service patterns that are to be operated on the tracks. T

Line complexity at junctions

Along this same vein, increasing the complexity of the routings of lines at a junction requires an increased complexity of the track layout to be able to handle those trains.

(example image)

Speeds and service patterns

In terms of raw throughput, the highest capacity of a one-way line can be achieved by ensuring all the trains have the same speed and stopping patterns. This can be seen in many metro networks and high capacity routes like the Crossrail core sections. The mechanism for this is displayed in figures 1 and 2, in part 1 of this guide.

However, in larger networks, it becomes necessary to have faster services that skip stops to retain reasonable journey times on the longest routes. Therefore the balance that must be struck is to minimise the different speed/service patterns of trains on a given stretch of track. Keeping a simple setup with only two service patterns usually provides a good balance, this can be seen on the NS network in the Netherlands, where there are effectively only two train categories (fast Intercities and slow Sprinters).

Where three different service patterns are present, it can be beneficial to separate them physically onto different tracks. High speed services could be added to the simple Intercity + Sprinter model by making these trains take separate lines for the high speed segments and following Intercity speeds and stopping patterns where they share tracks.

Passing loops at stations and timed overtakes can allow increasing the capacity of lines where multiple service patterns conflict, but often quad tracking these lines becomes necessary when running substantially more than 2 slow and 2 fast trains per hour.

Fast-slow separation

Route separated track near Zwolle. Note that the green line terminates and doesn't interact with the purple/pink lines on the other track pair.

Usually when quad tracking lines, they are split into pairs of fast and slow lines, this ensures that speed conflicts are minimised

A variety of different configurations can be used, each has their own advantages and disadvantages

Route paired quad track

Route separated quad track track sections are essentially two independent double track routes. It can be of use where the quad section of quad track is short and/or the two routes are to be operated independently.

In the above example at Zwolle, the trains on one of the track pairs go up to Meppel, while the other track pair goes to Emmen. The trains to Emmen terminate at Zwolle and have their own platforms at the station there, so they don't interact at all with the trains taking the route to Meppel (which originate in the directions of Lelystad and Amersfoort).

Using route separated quad track is generally not recommended if trains need to weave between the two routes as it leads to many path conflicts that hurt capacity or necessitate the building of flyovers. Additionally, if there is a mix of slow and fast trains on the routes, the capacity of two double track routes is inferior to speed separated track where the fast trains have their dedicated track and the slow/stopping trains have theirs. However, as covered below in the section "Weaving at quad track entry", short sections of route separated quad track can be advantageous in some scenarios.

Speed paired quad track

Speed paired quad track is one of the more common configurations that can be seen in real networks, particularly on longer stretches of quad track. Some places where it can be seen include the UK (GWML, GEML, Brighton Main Line, MML), Germany (most of the (Siegburg-)Köln-Düsseldorf-Duisburg-Dortmund-Hannover corridor, Karlsruhe-Offenburg, Augsburg-München, ABS Nürnberg-Bamberg (under construction)), Finland (Helsink-Leppävaara and Helsinki-Kerava)

It can be beneficial for creating simple junctions on one side of the line where only one of the track pairs needs to be able to branch off.

This advantage however only really exists for at grade junctions, for grade separated junctions, speed paired quad track will usually take up more space than direction paired (see for example. Also, if branching tracks join from either side of the main lines, extra bridges will be needed to avoid crossing the fast tracks. Furthermore, it is basically not possible for trains to easily go from the slow tracks to the fast tracks without creating a nightmare of path crossings or a complicated set of flyovers. For these reasons, the use of speed paired quad tracks should generally be avoided.

An example of this problem can be seen at Didcot on the GWML, where a fast train going from Reading to Oxford has to cross the opposite fast line, and both slow lines to take the branch towards Oxford. The single train is effectively occupying all 4 tracks for a span of 6 minutes if we assume a typical 3 minute headway before and after the train.

FU-FD-SU-SD Places tracks next to each other

Often less efficient due to being difficult to bring branches off on both sides

Can require weaving junctions with relatively large footprint

Impractical to allow trains to

Direction paired

FU-SU-SD-FD or SU-FU-FD-SD

Allows easy transitioning between fast and slow lines, much increased flexibility in service patterns (intermediate speed services can stay on fast lines most of the time and be moved over to slow lines to stop at stations and be overtaken by fast services)

Outer slows inner slows

Inner slows brings advantage of easy terminus/reversing stations for the slow lines, useful at the end of an s-bahn route for example. (Gouda Goverwelle, Houten Castellum)

Outer slows allows branching off the slow lines more easily, stations can take less space, can be easier in some situations for noise isolation

Table of different quad track groupings

Category	Route paired	Speed paired	Direction paired (fast outer)	Direction paired (fast inner)
Junction complexity vs capacity	Poor unless routes are independent	Poor unless at grade (still detrimental to capacity)	Medium, favours joining fast tracks which can be problematic, easy for maximum capacity	Good for grade separated junctions, favours joining slow tracks, easiest for maximum capacity
Support of different service patterns on a route	Poor (behaves like 2 tracks)	Mediocre (limited capacity to change between fast and slow tracks)	Good (trains can easily change between fast and slow tracks)	Good (trains can easily change between fast and slow tracks)
Ease of reversing trains	Good, separated by route, low path conflicts for both pairs	Good, low path conflicts for both pairs	Medium, very good enroute (easy to have suburban or regional services stop and reverse), poor at major stations (usually requires flyovers for maximum capacity)	Bad, only low path conflict reversing on fast tracks
Passenger usability	Cross platform transfers at best one directional	Cross platform transfers at best one directional	Cross platform transfers possible between fast and slow tracks	Cross platform transfers possible between fast and slow tracks
Station footprint	Large	Medium (requires platforms in between the tracks)	Medium (requires platforms in between the tracks)	Small (platforms between tracks not necessary for stops since they can be located on the slow tracks)

Y junctions

Y junctions are arguably one of the most simple track layouts where path conflicts can arise.

Quad track Y junctions

Weaving at quad track entry

As quad track lines typically operate at high frequencies, avoiding path crossings usually becomes paramount

Where a double track line diverges from a quad track line

Bringing the diverging tracks off between the outer and inner lines of the quad track section avoids traffic from the inner lines having to cross the outer lines when leaving the main line.

4+2 Weaved Y Junction

This requires very little extra construction, adding only one extra bridge compared to the simple layout, however the reduced space from placing the bridges in the middle of the other tracks can create challenges during construction. A very clean example of this configuration can be seen at Riekerpolder (NL).

Station approaches

Station approaches generally are quite complex

Being smart with platform assignment can reduce path crossings

A large distinction needs to be drawn between through stations and terminus stations, some cases stations are a mix with some terminating services and some through services