Route availability

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).

Network Rail currently gives the allowed axle loadings as follows:[2][3] Before nationalisation the Big Four railway companies had their own classification systems.

GWR's system for classification featured a coloured disc painted on the locomotive cab side to indicate its 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 RA 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 The withdrawn loco 01001 is just visible at the rear.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.