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The cab side On 1 July 1905 the GWR introduced a system for denoting the haulage capabilities and weight restrictions applicable to the various classes of GWR their locomotives. From mid-1919 this information began to be displayed on the locomotives from themselves, enabling railway staff to make a quick assessment of a locomotive's capabilities without the need to refer to tables of data. The display took the 1920s onwards carried form of a coloured disc bearing a letter. The colour of the disc showed the locomotive’s locomotive's axle weight classification, which determined which routes the locomotive could work on. The letter showed the locomotive’s locomotive's power classification, which was used to determine the maximum load it could haul on each route. The disks were initially placed high on the cab side but were moved to a position just above the cab number plate during the Second World War to make them easier to see during blackout conditions.<ref>Wikipedia</ref>
The axle weight was the main factor used in determining route colour, but this could be varied where it was considered appropriate. An example of this was the 5700 class pannier tanks, which were reclassified from "Blue" to "Yellow" by BR(W) in 1950. The reason given was the low 'hammer blow' that the class produced, which reduced the wear on the track.
additional info
[[File:7812_Cab_20130806.jpg|thumb|300px|right|Erlestoke Manor was rated axle load class ‘Blue’ and power class D]]
GWR locomotives currently or formerly resident on the SVR were classed as follows:
{| class="wikitable"
! Locomotive !! Type!! GWR Power class !! BR Power class!!GWR Axle load class
|}
Certain routes could be modified as "dotted", for example "Dotted Blue" meant a route could be worked by "Blue" locomotives but subject to a 25mph speed restriction. Locomotives in the lower "Yellow" and "Uncoloured" classes could work the route without this additional speed restriction.
===Severn Valley Railway Route Classification===
However in BR's Western Region, the old GWR system continued in use until after the Severn Valley Branch closed in 1963, with BR locomotive classes being allocated the relevant GWR route colours. For example a BR Standard Class 4 tank engine such as [[80079]] was given the general BR classification of axle load class RA5 and power class 4MT. Within BR(W) the route availability became "''Class 4 (2-6-4T) (80XXX): Prohibited on 'Yellow' and 'Uncoloured' routes. Restrictions – as for 'Blue' engines.''"<ref>[http://www.michaelclemensrailways.co.uk/?atk=559 ''British Railways Western Region, Locomotive Route Availability, June 1963'' via Michael Clemens]</ref>.
One result of BR's new method of assessing weight restrictions without solely relying on axle weight was that the 5700 class pannier tanks were reclassified from "Blue" to "Yellow" by BR(W) in 1950. The reason given was the low 'hammer blow' that the class produced, which reduced the wear on the track.
==Preservation==