Here's a teardown of the train. With the charger unplugged, set your multimeter to DC Volts and measure the voltage of the battery at the battery terminals. It should say on the side of the battery what the voltage is supposed to be. If the battery measures the correct amount, measure the voltage again with the black lead on the negative battery terminal and the red lead on the far end of the red wire. I would think it terminates at the back of the switch. If there is no voltage there then the in-line fuse is blown and will need to be replaced.
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