Unusual one last night on the up. Service was topped and tailed with two locos but the wrong one was at the front apparently. Multiple shunt moves at Truro to put it right which took 80 minutes and delayed the following Cross Country Service by over an hour.
Sleeper recovered most of its lost time by the time it reached London.
Excuse my loco ignorance, but aren't they both the same?
If both are in full working order and are of a suitable type, then yes they are the same and it matters not which one is which end.
In practice though a variety of faults and failures can render a loco available to assist in moving the train, but unfit to be at the front.
Examples include defective headlight, damaged windscreen,
TPWS▸ not working, radio not working, and in cold weather cab heater not working.
In other cases a sick but not yet dead loco might be attached at the rear in order to provide
ETS▸ (electric train supply for heating, cooling, lighting etc) The train can then be hauled by loco at the front, perhaps a freight engine not equipped to supply ETS.
Various other permutations exist. In many cases, both locos may be at the front, but due to a defect on one they have to be in the right order.