Zephyr
Hello & Welcome to the Forum!
Sorry to hear about your connectivity problem.  If you have access to a multi meter, check each piece of track to see where your power loss is. You might need to add another feed to that area.
 STOP USING Sand Paper on your track. IF you must, use heavy aluminum foil to rub the rail surface.
Clean your track, pins and recepticals with alcohol, smoke fluid, etc on a rag or paper towels. while you're at it, clean your pickup's & wheels on your locomotive.
That should bring you full circle, listening to the clickety clack as your train goes roundy round.
Happy Rails!
dave