An assisting engine is an additional locomotive attached to a train to assist the train engine – the locomotive assigned to haul the train. Assisting engines are added if the load of the train becomes too great for the train engine due to exceptional traffic levels, to negotiate steeply-graded sections of the route calling for greater tractive effort, or in the case of the train engine breaking down or experiencing difficulties.
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An assisting engine could be coupled directly to the train engine as a pilot engine, or it could assist from the rear of the train as a banking engine. A train headed by two locomotives thus coupled together was said to be 'double-headed'. Where an assisting engine acted as a pilot, it would normally be coupled ahead of the train engine, except on those railways – such as the old Great Western Railway in Britain – that required the train engine always to be in the lead. In which case, the assisting engine would have to be inserted between the train engine and the train.
Currently in the UK, where most trains are self-powered multiple units, assisting locomotives are kept at strategic points about the network to go to the aid of any train experiencing problems and in danger of blocking the line. These locomotives are familiarly known as 'Thunderbirds' after the TV series of the same name featuring 'International Rescue'; Virgin Trains's Class 57/3 engines are even named after characters from the series.