I think it's level, but a tad distorted. I would "crab in" the top of the frame on each side towards center, then crop accordingly...
I think it's such a small extent though, I am kind of surprised they rejected it. Especially when you consider how many blatantly obvious ones get through.
I wasn't that wide, but having both trains stretched out like that and so close to the edges of the frame I think made it difficult to see it level (maybe there was some minor distortion even - although I didn't see it).