my only recommendation would be given your stated limitations is to send to bachmann for repair service -  if it is new it should be under warranty -   i have heard their service is excellent but have no first hand experience
wonderful day to all

my only comment would be if you are using bachmann locos and if you are looking for something simple and yet functional , use the bachmann products -  in my opinion it is very difficult trying to solder all those little wires for a replacement products unless you have great eyesight and lot's of patience

if it is a simple loop of trains on the floor around the tree, i would recommend O-Gauge or G-Gauge toy trains -  however if you are looking for something more detailed then On30 is the best bet -  On30 is also very compatible with the popular ceramic christmas buildings - anything smaller is too fragile to stand up to the abuse of under the tree action - have fun

i just checked the grandt line website and the caboose is now listed as back in stock after a long period of being listed as not available - i had been watching it for some time now -hope that helps - perhaps your dealer may yet get it ?