Capacity of a transport line between two major cities

Note: this article continues the previous one:  Capacity of a transport line, again


The definition of a region

Before diving deeper, let’s define the object of our study. Today, a region is a subdivision of a country. While it varies much in size, we can admit generally that a region has on the average 100-200 km across, from a 100×100 km  square to a rectangle about 200×210 km.  Normally, in a region there is one city who is the largest. Often, there are 2-3  smaller cities who are relatively large by themselves. Between these cities, several dozen smaller towns  spread in across the territory.

The communication corridors

Usually, the large cities are connected by the rail network, with high speed trains mixing with low-speed regional or commuter  trains. There are several possible configurations:

  • the time controlled high speed track
  • the priority controlled high speed track
  • the reserve track
  • the fully separated tracks

The first two configurations require little investment in the infrastructure. The last two are based on some investment, going from moderate to high. We will see the configurations in the following sections, but before that, we must understand that based on many factors, sometime it is virtually impossible to achieve  the desired results (increase the capacity of the rail line) without substantial investment.

Note: We are completely skipping the one track corridors.

Time controlled high speed track


This is a very old configuration, dating back to the 19th century.  The classical line between London and Liverpool, part of the WCML network,  mixes fast trains with slow trains. The only way to achieve high speed is to maintain a velocity above a certain value. This is possible by using time prioritization.  The schedule of commuter trains is calculated in such a way that the low speed trains (40 km/h)  and the high-speed trains (200 km/s) never share the same track. This is very important as the express trains cover bigger distances than commuter trains, in practically the same amount of time. While this is important, nevertheless, the commuters are part of the local economy and the rail network around big cities is relatively well developed.

Priority controlled high speed track.

priorityThis configuration is similar to the time-controlled one. In order to have a high corridor capacity, many trains are on the segment at the same time, high speed express trains and low speed commuter ones. The trick is to ensure that the commuter is not slowing down the express. By using  an intelligent signaling system,   the commuters stop and wait for the fast train to pass.  While additional tracks are required in the local stations, this is not a problem. Most rail stations have additional tracks (platforms) and they can accommodate the commuters. Most of the rail networks in the world function according to the priority principle. In general, this approach puts much stress on the routing systems, especially the mechanical parts. Some of the biggest train disasters occurred because of mechanical failure  in the moving parts of the track infrastructure.

The reserve track


This is a very interesting case. The basic principle is that of the fire lane. An express train that need to pass in front of a commuter train will use a third track to achieve the desired outcome.  What’s interesting with this approach is the possibility to have trains moving in opposite directions on the same track, (separated by  a safety distance of  at least 20 km from each other).

The construction of a third track is possible in most of the cases, with the exception of high mountainous passages, bridges and tunnels.    The exceptional cases can be solved with local enhancements, which will not be discussed here. This configuration stresses the mechanical parts of the tracks, too, but the wearing is greatly reduced.

The fully separated tracks


Commonly known as quadruple track, this  configuration has a great advantage over all the other types of tracks. As he commuter trains and the express ones are physically separated for the full length of the tracks, the capacity of the corridor is not only doubled, but can be increased according to the capacity of the high speed trains. A big advantage of this configuration is the possibility to do maintenance on one express  tracks without affecting the traffic of the commuter trains and vice versa.

The quadruple tracks come at a cost. The width of the corridor is practically the double of the double track line. The cost of land increases and some terrain configurations forbid the tracks.  In such cases, a brand new corridor needs to be built, at an even bigger cost. Four track bridges are very expensive, too. Fortunately, traffic management can cope with it.



The increase of the population of a region must be anticipated and communication corridors must be adapted before it is too late. Unfortunately, while applicable to subway, the quadruple track approach is extremely costly and digging a supplementary pair of tunnels is very expensive.




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