I was doing some cleaning recently and found my copy of Handbook for Model Railroaders, 2nd edition, by W. K. Walthers. It was published in 1939 and has text that is extremely relevant to understanding not only rectifiers but also later electronic versions that are sometimes seen on OO models, as the vast majority of OO gaugers active and building layouts after WWII operated their OO models by Direct Current. Walthers explains,

The most satisfactory type of commercial rectifier is one which uses copper oxide as an electrolyte for rectification. These rectifiers are built up of copper wafers which have a layer of copper oxide baked on one face. Such a combination offers a low resistance path between copper and oxide and a high resistance path from oxide to copper. Hence, current will pass only in one direction (the leakage current is so low as to be negligible).

These treated copper disks are stacked together on insulated bolts. Since each disk has a certain carrying capacity and permissible voltage drop, the number of disks in a stack will depend on the voltage required and the number of stacks connected in parallel on the current needed. The arrangement of disks is similar to batteries connected in series multiple. Leads are attached to the disks for A. C. and D. C. connections. The A. C. will flow only one way through the disk and so we obtain a pulsating current equal to the cycles of A. C. applied.

There is a bit more in this section but how this is useful to the OO gauger of the time comes up a couple pages later, after the discussion of motors.

In the smaller gauges, especially HO, D. C. power is nearly universal and is used with permanent magnet motors which reverse or changing direction of the power supply, no relays being needed. Bridge rectifiers are also used with ordinary O gauge series wound motors to give the same ease and simplicity of control.

In a more modern, toy train context the relays he speaks for A. C. would be an E-unit reverser.

When OO was developed essentially it was designed to a point around the smallest motors then available and those were A. C. But D. C. operation really simplifies things and was the type of operation initially offered by Scale-Craft in their earliest production run of their 4-6-0. However, recognizing market forces they had to back up and offer their Universal motor with options to wire it for manual reverse for A. C. or a rectifier for automatic reversing on D. C.

By 1941 Scale-Craft also marketed a bridge rectifier. This wiring diagram for the “reversing rectifier” is presented on page 91 of the catalog, and the catalog description states

The use of the bridge rectifier for polarity reserving with direct current is the latest development in remote control practice. It is trouble proof, simple, and cheap to install. These little rectifying units are placed between the motor field and line; they preserve a constant direction of field current, regardless of line reversals. The result is, that a line reversal reverses the motor—the same as with a permanent magnet outfit. It must be remembered, however, that the bridge rectifier cannot be used with alternating current. It is made for direct current only, and will be burned out in a few moments if used with an alternating current.

Time moved on. Bill Chapin showed me several models he had modified to use a modern electronic rectifier from Radio Shack that ran great with the original Scale-Craft motor.

In any event, if you see a stack rectifier on a locomotive it is set up for D. C. operation and if you see something electronic hooked up to a vintage motor it probably also is a rectifier for D. C. operation, not an E-unit A. C. reverser.