Definition of "unsteadiness"

The Marsh rebuilding, with the higher-pitched and larger boiler and new standard boiler mountings, completely altered the appearance of the engine, which in its last guise was a very handsome machine; but unsteadiness in running at speeds over 50 m.p.h. resulted in no further such conversions being made.

1942 March, “The Why and the Wherefore: A Rebuilt Brighton 0-4-2 Tank”, in Railway Magazine, page 95