When WS introduced their system a few years ago I harassed/badgered 
another forum member to open one up and post photos here so that I could see how they did it. Assuming that's what we're talking about then you can use a 5 cent resistor to limit the current into the pre-installed LED lights. Perhaps someone has already done this and can offer suitable resistor values. Or, depending on what you have on your bench (meter, bag of random resistors, DC train transformer, etc.) we can come up with a solution.
You don't want to use a train power pack without some kind of mechanism (i.e., a resistor) to limit the current flowing into the LEDs. A tiny change in voltage applied to an LED can result in a large change in current which will momentarily seem brilliant until the LED dies.
