Whistle/horn switch on Lionel KW Transformer

I'm afraid we have been blindsided by the evolution of technical terms.  The term that Lionel used for what I called a "diode" was "rectifier".

A "diode" was originally a vacuum tube with two elements, used as a rectifier.  (A "triode" had 3 elements, a "tetrode" 4, a "pentode" 5.)  There were also solid-state rectifiers, like selenium and copper-oxide, the kind that Lionel used.  With time, germanium and silicon rectifiers replaced vacuum tubes and came to be called "diodes".  (The "reference designation" for them on schematics changed from "CR", meaning "crystal rectifier, to "D".)  The term "rectifier" is now used in a more general sense,  as a circuit that rectifies, and may include multiple diodes and other components to accomplish that task.

The symbol for a diode (in the modern sense) is 

Current flows in the direction of the arrowhead.  The left terminal is the "anode", the right terminal is the "cathode".  The cathode is often marked by a ring around the diode's body.  (Diode symbols are reversed on Lionel schematics.)