Power enters through the tender. You must have the tender and the engine both on tracks and linked with electric cable.

Check the power circuit that goes to the track if there is an open circuit, open circuit means, break or cut line that can not flow the current going to the track to give a power to the train, ok.

The track clip did break at the power source (repaired). However the train would run and stop . By checking with a voltage meter I found voltage to be spuratic. If you examine the second car it has spring loaded contacts (brushes) that actually ride on the track. This will leave a residue on the track . Clean the track with some fine steel wool and your back in business.
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Wheels will not turn when train is on track

I have had my track track clip break at the track connector. Had to repair it by wrapping it with some small wire to make contact with the track again.