3 locomotives in the 1980s the 37s were selected as the standard Type 3 and many of the fleet were given a heavy overhaul to prolong their life into the 1990s and beyond. Some were fitted with electrical train heating (ETH) equipment in the 1980s to become the 37/4 sub-class, initially for use on the West Highland Line, the Welsh Marches line and South Wales–Bristol area services and Far North Lines but later seeing use in north/mid Wales and occasionally the West Country. In 2010, they were used on passenger services on the Cumbrian Coast line and Wherry lines. Rebuilding A number of locomotives were rebuilt as Class 37/9s in the late 1980s to evaluate Mirrlees and Ruston engines for possible use on a new Class 38 freight locomotive. These locos were heavily ballasted to improve traction and had excellent load-hauling capabilities, but the Class 38, understood to be a 'modular' locomotive based on the approach that gave rise to the Class 58 diesel loco and the proposed Class 88 electric loco, was never built. Axle load The Class 37 has a relatively low axle loading for its size and power. With the withdrawal of most of the smaller types of diesel locomotive, this left it as the only mainline type available in significant numbers for lines with weight restrictions, and for a number of years 37s handled almost all locomotive-hauled services on the West Highland Line, the lines north of Inverness (Far North Line) and in parts of Wales. The Class 37 has Route Availability 5 and this is one of the main reasons it is still in use on the network. Note that class 37/7 and 37/9 have an RA of 7 due to their extra ballast weights. Regional variations There are several differences between particular locomotives, some of them easily seen. Western Region Class 37s can be identified by 'cow horns' around halfway up on the outer edge of each end of the lamp brackets. When British Railways took over from the Great Western Railway the use of Great Western lamps continued. Their brackets used an L-shaped upright that was parallel to the direction of travel, unlike the other regions which used transverse brackets. Another difference between the regions is by the nose end headcodes. Lower-numbered, split-box Class 37s were allocated to northern England and east Anglia; centre-box locomotives were almost all allocated to Wales and the