single-wheeler

single-wheeler (plural single-wheelers)

1. (rail transport) A steam locomotive with a single driving axle.
   • 1941 February, O. S. Nock, “The Locomotives of Sir Nigel Gresley: Part 1—G.N.R. 1911-1914”, in Railway Magazine, page 77:

     It links the time when single-wheelers were still being used on crack expresses to our modern streamline age, when up to the outbreak of war developments both at home and overseas were following thick and fast upon each other.

This term is a misnomer, as each locomotive had at least four wheels, usually more.