(this is related to the topic on Power buffers in DCC forum)
I think Trev is stopping his trams by removing 12V power from a section of track. This will result in an instant stop. As he has a fixed polarity on the rails (ie one direction only), he could try a big capacitor like this cct:
The diode is to protect the cap from reverse connections of the power supply.
I tested a single capacitor supplying a motor ? I had a 47000uF, 5V cap and used it to supply a motor with its wheels in the air, taking 140mA. The voltage decay when supply was removed was more exponential in shape and the motor took about twice as long to stop as theory predicts.
The calculated value in this case is:
C (Farad) = motor current (A) x time to maintain supply (sec)
acceptable voltage drop in this time
But testing shows the capacitor probably only needs to be ½ the calculated value.
You could probably incorporate this in each of your circuit boards. Unfortunately it only gives a soft stop not a soft start.
Here's the CRO trace of the test above.