Remember your post about blown "thing" across the Neqida motor you had on test? Attached a pic showing that it is actually an electrolytic capacitor and abviously not meant to have reverse voltage across it! they are polarised! that explains it. First thing I did was snip mine off... Now struggling to find out what current/voltage the motor is as the train came with a 6 battery pack inside the boiler.
Mine cane with NiCad 7.2 I think, my testing was with an 18v Battery and my Battery DCC setup testing rig. I was careful not to shove Max Speed into the thing During testing and load was only the wagin carrying the battery and test rig.
It depends very much on what you intend to do with it. I would think that a 12v supply would not hurt it so long as you are not shoving max volts (max speed if you like) or pulling massive trains that would likely up the amps.
If you are going to put any RC into it you could minimise the volts by CV if DCC or by Output with a Deltang setup. Not sure if this would help with the amps if you shoved a big heavy load behind it. Some other bright person will no doubt comment on that statement. Also as these locs have plastic wheels, lightweight and cheep build I would be tempted to use it on lightweight Trains. Though a consist of same build wagons or coaches unmodified as supplied would probably be fine in a largish consist as they are as light as a feather.