What is "pony truck"

Bissell truck \Bis"sell truck\ A truck for railroad rolling stock, consisting of two ordinary axle boxes sliding in guides attached to a triangular frame; -- called also pony truck.

A pony truck, in railway terminology, is a leading truck with only two wheels.

Its invention is generally credited to Levi Bissell, who devised one in 1857 and patented it the following year. Hence the term Bissel bogie (spelt with one 'l') or axle is used in continental Europe. In the UK, the term is Bissell truck with two 'l's.

Conservative locomotive builders in Bissell's native United States did not take to the design, and it was not implemented until the Eastern Counties Railway in the United Kingdom fitted one to their No. 248 in 1859. Pony trucks of similar design became very popular on British locomotives thereafter.