Let's put the DC motor theory to rest, now that we have a photo of the engines. They are MPC from the mid to late 70's and run on AC, which is what the KW puts out. For the record, the value of those trains is primarily sentimental. The monetary value isn't that much, and the pieces are fairly easily replaced. As was mentioned earlier, they are also easily repaired.
Do you have a Volt - Ohm meter? If you don't you can get one for less than $10. Very handy tool, especially if you want to play with trains.
There may be a short in a single piece of track. This can happen if one of the fiber insulators on the center rail is damaged. It only takes one, and it may not be obvious from visual inspection.
First, disconnect the track from the transformer. Then, using the meter on the Ohms (Ω) setting, touch the center and one outside rail with the probes. If the reading is zero, you have a bad piece of track. To locate the bad piece, open up the layout in two places, cutting it in half. Test each half. Repeat this process until you find the bad piece. Just remember that there may be more than one bad piece. This "binary search" method is still faster than checking every single piece.
Sparking wheels and rollers are usually a sign of dirty track or the rollers themselves.
