The engine was enclosed so that it would look like a passenger car, hence the name 'dummy'. The story goes that if the horses saw the engine they would get spooked. It was thought that the more familiar appearance of a coach presented by a steam dummy, as compared to a conventional engine, would be less likely to frighten horses when these trains had to operate in city streets.
Later it was discovered that it was actually the noise and motion of the operating gear of a steam engine that frightened horses, rather than the unfamiliar outlines of a steam engine.
Compiled by Dr. Neil Gale, Ph.D.