A cart used in mines and on railways. A railway trolly is worked by the hand, which moves a treadle; a coal-mine trolly used to be pushed by trolly-boys; ponies are now generally employed instead of boys.
(Welsh, trol, a cart; trolio, to roll or trundle, whence “to troll a catch” —i.e. to sing a catch or round.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894