Excellent response…

If you do have radio control, you’ll have to take the “dead” loco apart and find the receiver. If there is no frequency label on the receiver, there should at least be a manufacturer and part number that you can look up. Are there any frequency selection switches? Perhaps the former owner set two locos to work together on one train. To determine if the problem is in the receiver or not, you’ll have to bypass the receiver and see if the loco runs on current from a standard Lionel transformer. If the loco doesn’t run with power from a transformer, you’ll have to trace the electrical connections from beginning to end, from the track contacts to the brushes on the armature. Old soldering joints often fail, wires break inside their insulation, brush springs break, etc..

Trouble-shooting why a loco doesn’t operate as it should is not something that can be determined by wondering why. It takes sometimes tedious investigation. It takes dividing the problem into sections and investigating each one until the cause of the problem is solved. Since your transmitter works with two locos, it is probably not the problem. So is the receiver faulty? Is the wiring in the loco faulty? Are the settings, if any, on the receiver mismatched? Or might it be something as simple as moving the selection switch on the loco from the constant position (the loco continues as before each time the current comes back on) to the alternating position (the loco cycles through forward-stand-reverse-stand as almost all those Lionel locos did), or is that selection switch in the half-way position so that nothing works?

Many of us on this blog would like to help, but there’s just not enough detailed information for us to go on.