donkey engine

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A small auxiliary steam engine used for hoisting or pumping, especially aboard ship.
• noun A small locomotive.

from The Century Dictionary.

• noun In machinery, a small steam-engine used where great power is not required, and often to perform some subsidiary operation.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (nautical) a small engine (as one used on board ships to operate a windlass)
• noun a locomotive for switching rolling stock in a railroad yard