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:
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.
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.
(example image)
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.
(example image)
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.
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, because the two trains are 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.