Well.... it's not RF propagation, that's for sure. At 455 KHz the wavelength is 2160 ft so unless the locomotive antenna is that long, it's going to be dominated by E-field capacitive coupling modes. "Signal" is the AC current flowing through through the locomotive, and through the capacitance formed between the engine and the building.
It's like when you put your hand over a TMCC locomotive that is struggling with signal and suddenly it starts to work. Your hand is like the second plate (building side) so you're lowering the D (distance between plates) parameter in the capacitance C=EoEr A/D. The moisture is taking the place of your hand, increasing the capacitance through the Er parameter instead of the D parameter.
