Works test train coaches were part of the engineers fleet, though often retaining passenger livery these coaches were not for passenger use but used as a load for controlled testing of overhauled locomotives. The interiors may be unchanged, stripped out or replaced with weights to provide a known load, a newly overhauled locomotives' performance could be checked with a series of controlled test runs before the loco was returned to service. An excellent excuse for an unusual locomotive to turn up 'on test'!
Following the overhaul of a diesel locomotive at BREL Doncaster Works and after static testing on a load bank, a test run with a suitable load would be arranged. Typically the test train would run between Doncaster and Tyne Yard at high speeds to make sure that the engine and all other control equipment operated correctly, only once a successful test run was completed would the locomotive be released back in to traffic with British Rail.