Hi Warren,
As far as taking apart locos goes, it's not a "one process fits all" deal. Different manufacturers assemble their locomotives differently, so taking one apart is dependent on who made it. Then there are differences even within the same manufacturer since each type of engine is different.
Now about your wheels:
Are you sure the wheels are spinning at all? Maybe the motor is going, but the gears are broken or stripped. When you power on the loco, look closely at the wheels to verify that they are really spinning. If they don't, then you will need to take apart the engine to fix that problem.
Are the wheels grooved? Depending on who made your particular engine, the wheels may sometimes have traction tires. They look like rubberbands wrapped around the wheel, so that the engine has some "grip" on the track. If they are missing, then you'll know because the wheels will have a groove in them. If this is the case, then you simply need to put on some traction tires.
And as Selector stated, maybe something from the engine is getting hung up on the track, pinning the engine in place. Just follow his advice to check for that type of problem.
If you have the ability to post pics, maybe someone here can identify what kind of engines you have and maybe even help you with the disassembling part. Let us know and good luck.
