Hi I have RC Old Timer Train (set 4017 / 4019 / 4021).
If I leave the locomotive's battery case in overnight, the battery is dead the next morning.
But, if I remove the battery case after I'm finished running the train, the battery stays good for years (two years going, now).
I can leave the batteries in the remote control without a problem. Leave the battery in the locomotive, though, and dead the next morning.
-Tim