An assisting engine is an additional locomotive

Locomotive

A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...

 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

Tractive effort

As used in mechanical engineering, the term tractive force is the pulling or pushing force exerted by a vehicle on another vehicle or object. The term tractive effort is synonymous with tractive force, and is often used in railway engineering to describe the pulling or pushing capability of a...

, or in the case of the train engine breaking down or experiencing difficulties.

Pilot or banker

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

Double-heading

In railroad terminology, double-heading or double heading indicates the use of two locomotives at the front of a train, each operated individually by its own crew. The practice of triple-heading involves the use of three locomotives....

'. 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

Great Western Railway

The Great Western Railway was a British railway company that linked London with the south-west and west of England and most of Wales. It was founded in 1833, received its enabling Act of Parliament in 1835 and ran its first trains in 1838...

 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.

Thunderbird

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

Thunderbirds (TV series)

Thunderbirds is a British mid-1960s science fiction television show devised by Gerry and Sylvia Anderson and made by AP Films using a form of marionette puppetry dubbed "Supermarionation"...

 featuring 'International Rescue'; Virgin Trains

Virgin Trains

Virgin Trains is a train operating company in the United Kingdom. It operates long-distance passenger services on the West Coast Main Line between London, the West Midlands, North West England, North Wales and Scotland...

's Class 57/3 engines are even named after characters from the series.