The Lionel e-unit was a wonder of ingenuity in the 1930s. But how does a Lionel e-unit work?
Open-frame electric motors like the ones Lionel used in its glory days have two major parts: a field and an armature. Both of them are electromagnets. The field stays stationary, while the armature turns. The armature turns because its polarity constantly changes.
The field’s polarity depends solely on wiring. Reverse the wires on the field, and the motor runs the other direction. It’s simple.
Changing the polarity remotely isn’t quite so simple.
The Lionel e-unit changes the polarity of the field each time you sequence the power. It consists of a solenoid, a series of copper fingers, and a drum with a ratchet and a series of contacts on it.
When you interrupt the power, the solenoid sequences the drum via the ratchet. The contacts on the drum engage the copper fingers, which changes which wire goes where. This changes the field’s polarity, or disengages it entirely to put the locomotive in neutral.
It’s an extremely clever design. When you think about it, the design makes a lot of sense, but it took decades for someone to think of it. Lionel’s rival Ives invented it in 1924, and Lionel couldn’t come up with something better. The story of Lionel’s buyout of Ives is complicated, but Lionel bought Ives to get that e-unit. Lionel found a way to make the Ives design smaller, but needed the Ives patents to make it legal. How Lionel protected the Ives trademark is another story.
Arcing can cause the e-unit to malfunction. It causes the copper fingers to get dirty. This hurts conductivity and causes heat. Eventually the heat can distort the fingers or the drum, eventually causing enough damage that the e-unit won’t function and the train won’t run.
Generally this process takes years or even decades, so this wasn’t a big problem for Lionel. American Flyer’s counterpart component was crankier. Marx’s counterpart component was far more reliable but lacked a neutral position. So the e-unit really gave Lionel a competitive advantage for several decades.