Welcome to the Auerbach and District Light Railway carriage works department:
Above is one of my green coaches, with the roof removed, to allow me to fit carriage lighting.
Hre you see the wiring
which connects a small battery unit to three leds. The black wires are all connectd together, similarly the red. I am clearly not an expert with the soldering iron.
All finished and it works!
And with the roof back on.
Here is a video of a night run to get the best effect.
Proof of the pudding 
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