Engine runs fine with the tender off the track but once the tender is on the track everything goes dim and it wont run. I believe its a short but can't find the anser
If you have done nothing to the loco and tender, it sounds like one of the tender trucks has swiveled 180 degrees which will cause a short. My 4-6-0 trucks can swivel 180 degrees.
Some Spectrum tenders cannot swivel this much but some can.
Rich