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If you mount the transmitter just above the solid metal of the caboose top (similar to a metal roof), you’ll get a capacitive effect between the caboose and the transmitter, and you might not need to attach an RF ground wire at all (thus eliminating one potential legality issue, i.e., grounding to something similar to an elevated mast). You’ll still need to provide some sort of lightning protection.
I once had an FM transmitter mounted about 6 feet above a metal roof. When I was forced to mount it within a few inches of the roof (with no ground attachment), I noted a significant improvement in the signal quality and range; I would think the same principal would apply, maybe even more so, for an AM transmitter.