Capacity, in this context, is the ability to reserve infrastructure elements to allow the passage of a train.
Capacity is expressed in both space and time: the reservation of an element can block a specific zone that becomes inaccessible to other trains, and this reservation lasts for a given time interval.
It can be displayed on a chart, with the time on the horizontal axis and the distance traveled on the vertical axis.
Example of a space-time chart displaying the passage of a train.
The colors here represent aspects of the signals, but display a consumption of the capacity as well: when these blocks overlap for two trains, they conflict.
There is a conflict between two trains when they reserve the same object at the same time, in incompatible configurations.
Example of a space-time graph with a conflict: the second train is faster than the first one, they are in conflict at the end of the path, when the rectangles overlap.
When simulating this timetable, the second train would be slowed down by the yellow signals, caused by the presence of the first train.
A train slot corresponds to a capacity reservation for the passage of a train. It is fixed in space and time: the departure time and the path taken are known. On the space-time charts in this page, a train slot corresponds to the set of blocks displayed for a train.
Note: in English-speaking countries, these are often simply called “train paths”. But in this context, this name would be ambiguous with the physical path taken by the train.
The usual procedure is for the infrastructure manager (e.g. SNCF Réseau) to offers train slots for sale to railway companies (e.g. SNCF Voyageurs).
At a given date before the scheduled day of operation, all the train paths are allocated. But there may be enough capacity to fit more trains. Trains can fit between scheduled slots, when they are sufficiently far apart or have not found a buyer.
The remaining capacity after the allocation of train paths is called residual capacity. This section explains how OSRD looks for train slots in this residual capacity.