If new and defective out of box, I would contact the place you purchased it from.
They should be willing to help, or even exchange if possible. (Responsible, reputable places will.)
If used (as is, was is, yours is) then maybe see about parts. If it is easy enough to fix, parts only might be cheaper. Otherswise, is shipping fee to manufacturer more cost effective?
Last resort - Paper weight.
I once received a NIB Bachmann Spectrum diesel where they somehow missed a very important screw during assembly, but the shop I purchased it from was able to supply a replacement screw and fix easily. (This was clear back when I was 14, 21 years ago.)
Only other NIB issues I’ve ever had was defective factory decoder, and defective lighting in another from factory. One was replaced by manufacturer no charge, the other I gave up fighting over it and ordered parts to fix it myself. (And I make a point to not purchase anything else from that seller any longer.)
So, I will repeat the question - What is defective about it? And which model is it? The popularity of Bachmann units being as it is, someone might have come across the issue and it was simple enough to fix so cheaply they just did it themselves and can advise.