.you do see that the section of track the trailing truck and tender is sitting on is slopped downward at the joint right? That is causing the rear of the engine and the front of the tender to slope downward towards each other....in that particular photo that is.
I think the photo is slightly tilted, not the track. Regardless, the track is perfectly straight: put a ruler along it, or another straight red line as I just did with min image here, and it shows the track is perfecty even. The loco slops down at the back, and the cab is too low relative to the tender. I haven't taken apart the J, but I have taken apart enough Legacy steamers to know that inserting spacers at the rear to raise the dab end would not disturb the rear drivers off the rails on anhy of those. I seriously doubt that would be an issue. I would certainly try 1/8 inch of washer os so before doing anything else.
