The speed controller is called a No. 81 Controlling Rheostat made by Lionel Corp.
The speed control was made from 1927 to 1933 and lists for as high as $6.00 in the Greenberg price guide for 2006. How to wire it I am not sure as I have not seen this device before. Does it have two or three binding posts?
Separate rheostats were not made after the 1940’s as most were built into the transformers.
Lee Fritz
It has two terminals. Wire it in series with the transformer secondary. Rheostats were not built into any transformers to my knowledge. Instead, the voltage is adjusted by a wiper on an exposed secondary winding.