Route availability

With its relatively high axle load, the Class 67 locomotive has a somewhat limited Route availability of 8
A Class 86/2 No. 86 252 at Birmingham New Street railway station. This class has a Route availability of 6.
A Class 37 in British Rail large logo livery at Muir of Ord railway station, 1988 with a medium Route availability of 5.
Class 14 No. D9526 (as preserved) at Williton on the West Somerset Railway a low axle load and less limited R.A. of 4.
Class 01 shunter 01002 inside the shed at Holyhead Breakwater. The withdrawn loco 01001 is just visible at the rear. They had an RA of 1, and thus had the fewest axle load related restrictions put upon them.

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 a high RA 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.