What would be the most common reason that the interior lights no longer work on my Wonderland Express?
If it is only one car, I would guess that one of the wires has broken where it connects to the truck. There is one wire to each truck (opposite sides) held on by a small brass loop. Sometimes the flexing of the wire as the train goes around curves will cause the wire to break right where it attaches to the brass loop.
Paul
Not familiar with this set but it might be as simple as one of the trucks may be turned 180ยบ around and the pickup wheels are now on the same rail.
My experience with the passenger cars is that the solder connection at the truck usually comes loose! This has happened on a number of my Jackson and Sharp passenger cars. Not a particularly easy fix as you generally have to disassemble the car in order to gain enough wire for a resolder. While you are at it, reinforce the solder joint at the other truck, it will save some future grief!