Route availability

Route Availability (RA) is the system by which the permanent way and supporting works (bridges, embankments, etc.) of the railway network of Great Britain are graded. All routes are allocated an RA number between 1 and 10.
Rolling stock is also allocated an RA (again between 1 and 10) and the RA of a train is the highest RA of any of its elements. The train must have a route availability (RA) lower than or equal to the RA of a line to be allowed to use it. The RA is primarily related to the axle load of the vehicle, although axle spacing is also taken into consideration. In practice it is the locomotive which governs where trains may operate, although many high capacity 4 axle wagons have high RAs when fully loaded. (When considering the operation of trains the loading gauge must also be considered.)
The system was first devised by the London and North Eastern Railway, and perpetuated by British Rail to ascertain which locomotives can work on which lines throughout the rail network in Great Britain.
Exemptions may be obtained to allow locomotives to operate on lines from which they may otherwise be banned. An exemption might be granted by placing a speed restriction over a weak bridge, for example.

Line calculations

The route availability for a line is calculated by taking into account bridge strength, track condition, structural issues and so on.  A route availability of one (RA1) is the most restricted line, open to possibly one type of locomotive specially designed for it.  A route availability of 10 is the most open, usable by any locomotive that fits within the GB loading gauge that has been 'passed' for it (checked for conflicts with infrastructure such as platforms).

Historical notes

Before nationalisation the Big Four railway companies had their own classification systems.