WHAT DOES THE CAP DO WITH DC?
When working under plain DC power and capacitor can perform it RFI function with no side effects with respect to the motor control. In this case, the only source of RFI energy is the motor itself which is why the circuit exist in the first place. It has no effect on the DC power packs. Since more DC locomotives are sold than DCC locomotives, the majority of the market has no problem with the fitted RFI filter.
WHAT DOES THE CAP DO WITH PWM USED BY DCC DECOERS?
When working under DCC, the motor is no longer driven with plain DC power. Decoders which directly drive the motor now use what is called Pulse Width Modulation or simplistically "Pulse Power" to control the motor. Any type of motor power pulsing looks like RFI to a capacitor. This gets even worse when you use High Frequency (Silent/hyper) Drive decoder whose pulse drive even looks MORE like RFI to the capacitor. Since the capacitor attempts to "short out" any RFI looking signal regardless were it comes from, the DCC Decoder will see a momentary short circuit at the very beginning of each motor pulse!
Assuming the decoder still functions at all, it must deal with the short circuit by providing more current. More current = More Heat.
Hence it has the long term potential too hurt a DCC decoder even if it works.
PWM VS CAP TECHNICAL EXPLAINATION
The part that is causing the partial short circuit is called a Capacitor. Capacitors have a value called capacitance. To learn more about capacitance, go here:Capacitance
That capacitance value is involved in determining how much current the capacitor will draw.
I = C*dv/dt
To learn more about this equation, go here: Capacitance
This equation says that a capacitor will draw current when the voltage changes across its terminals. For RFI purposes the capacitors are electrically wired in parallel with the motor terminals. So what every power source is driving the motor is also driving the capacitor too.
Decoders use PWM motor control because it is
1) extremely efficient (minimum heat generation)
2) allows the use of very small parts (build small decoders)
3) can be easily controlled by the decoder's microprocessor (the device that makes the decoder work).
To learn more about PWM motor control, go here: PWM Motor Drive
V(DC) and I(DC) are terms representing voltage and current when DC is present. DC is any voltage or current that constant with time and has no frequency.
V(AC) and I(AC) are terms representing voltage and current when AC is present. AC is any voltage or current that changes with time and has a frequency.
PWM motor drive use a stream of pulses to drive the motor. The motor voltage is NOT constant. A typical motor pulse applies full track voltage (dv) for a very short time (dt) to the motor terminals. The full track voltage is removed from the motor terminals at the end of the pulse. So there is a change in voltage over a period of time. Motor pulse are a form of AC.
The above equation means the capacitor will pass current between it two terminals when the voltage changes. Stated another way, the motor capacitor draws AC current independently of what the motor draws.