Is DCC AC or DC current? - Model Railroader Magazine

A diode would work great - if you wanted full speed all the time. Decoders actually use a circuit called an H-bridge (since in a schemetic it looks like an H) and pulse-width modulation (PWM) to drive the DC motor from the DCC square wave. Most command control systems before DCC also used some sort of pulse system - because switching DC is HIGHLY inefficient and requires far larger components. At low speed, the motor might be getting 2 volts. Out of, for DC, 12 volts. That 10 volts is dissipated in the components as heat. It takes a good size heat sink that gets quite warm to handle even 1.5 amps - not tomention the 8a nd 10 amp large scale decoders! PWM allows the control devices to run either fully closed, or wide open. Minimal heat buildup because they are either passing NO current, or passing the load current with little drop (there's some - all semiconductor junctions have a voltage drop, but compared to droping 10+ volts, it's nothing). As the name implies, the width of the pulse is varied - the shorter it is, the less 'on' time there is, and the less average DC volts the motor sees. The longer the pulse, the longer the 'on' time and the higher the voltage the motor sees, and the faster it goes.
There IS a rectifier on a decoder, and a voltage regulator - to power the processor chip and produce fixed DC for the function outputs.

--Randy