Here are just some thoughts on this question. By 1950,
Burlington management was considering replacing the old, worn out, steam
engines in commuter service with diesels. There was a concern as to how the
diesels would perform, in the start-stop service. So why not use a surplus (for the Q) engine in
a test?
There is a well known characteristic of reciprocating steam
engines, stationary or mobile, that they have maximum torque (read acceleration)
at zero speed. Series wound DC motors (like traction motors) share this
characteristic. However, locomotive traction motors are not purely series wound
motor. If they were, only low speeds would be possible. To overcome this,
locomotives have “transitioning”. The E-5’s had manual transitioning, which made
them perfect for the experiment. Later Q E-8’s and E-9’s had their
transitioning modified for the stop-start operation. Anyone who ever rode an E
unit powered dinky knew the engineer could really “pin her ears back”. So it
wasn’t as easy as just taking an off the shelf E unit and putting it in
combination service (my dad used to say “Nothing in this world is ever SIMPLE”).