There are several different ways of defining the capacity of a railway line.
At its simplest, capacity is usually measured by calculating how many trains can pass between two points in a one hour period, and is thus measured in “trains per hour”. (Note that we are considering the capacity of the section of line in terms of how many trains can run upon it, rather than the capacity for passengers or freight on each individual train. That is a related but entirely separate issue).
So if you take two points on a main line, and calculated how many trains could pass between those two points whilst travelling at the maximum possible speed, that would give you a figure of (say) 30 trains per hour (tph), or a train every 2 minutes. This time between trains is called the “Headway”.
That’s all right so long as the signals are all evenly spaced, and there are no intermediate stations. But what if we look at a longer section, which includes stations at either end and also intermediately. Now the ability of the stations to handle the number of trains becomes crucial, especially as there will be conflicting movements at the platform ends as departing trains conflict with those which are arriving.
What if some of the trains need to run non-stop, whereas others need to call at the intermediate stations? The two different train types will consume a different amount of capacity. If we take the non-stop train as the “standard pattern”, then we can say that it consumes one path. Assuming the train headway is still 2 minutes (see above)then we can construct a scenario where a stopping train takes one minute to decelerate, spends two minutes in the station, then takes a further one minute to accelerate back up to maximum speed. So stopping that train once has cost two train paths. (4 minutes additional time equals two x 2-minute headway train paths).
Now take a look at this problem again, this time from the viewpoint of the operator of the stopping train. If most trains on the route are stopping services, we can adopt them as the “standard pattern”, so each one consumes only 1 path. Now it is the non-stop train which consumes additional paths.
Of course, if the route is shared with freight services, they will have an entirely different speed profile, and so may consume a great deal of capacity compared to the passenger services.
So why is this important now for Turkish Railways?
With the forthcoming division of Turkish Railways into separate Infrastructure and Operations businesses, along the European model, capacity is at the heart of the relationship between the two sides. Capacity is the commodity which is sold by the infrastructure owner to enable the train operator to run trains.
So the infrastructure owner needs to understand what the capacity of each line truly is. Otherwise it would be like the manager of a warehouse not knowing what it contains. The train operator must be sure of how much capacity each train is using – and it is likely he will be charged on that basis.
Also, punctual operation of the trains becomes much more important, particularly if competing train operating companies share the same tracks. For example, if I am operating a premium freight service from Baku to Frankfurt with timed deliveries, I don’t want my train to miss its booked path because the stopping passenger train in front is running late as a result of queues at the ticket office.
So it is usual to allow for a certain amount of slack in the system, (the UIC recommends 25% minimum) to cater for train punctuality and performance issues on main lines with different types of trains.
In this brief look at capacity, we have only just touched the surface of the issue. It becomes much more complicated once you start to include stations, individual nodes, networks, and differing types of trains and signalling systems. There are various ways to calculate the capacity, according to the circumstances.
Understanding what capacity is, how it is measured, and how it is traded will become central to the new TCDD organisation.
Photo: Jeff Hawken ©
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