In the opening, a slot machine is used by the main character. The slot machine is the same one used in The Fever (1960), in which an older gentleman is obsessed with the machine.
The title refers to the propulsion unit for a transportation system, such as the locomotive on a train, the tractor unit of a semi-tractor-trailer, the motor on a locomotive or railroad engine or the truck that tows a artillery gun. Also, in philosophy, a self-caused being that is the cause of all things.
This episode aired March 24, 1961, but the calendar on the wall in the diner is from December 1955.