Build_and_Break:
something that can handle transmitting the pressure readout through up to a mile of metal train, essentially with no line of sight at all without drawing an obsessive amount of power (because this must be portable and it will run on a battery), not bear any antennas over a foot long (again portability), and adhere to all FCC regulations.
I suspect that is virtually impossible.
Can you use a cell-phone / mobile phone system to act as an intermediary?
Does the railroad / railway company have its own wireless system for communicating with the drivers of trains? Could that be used as an intermediary?
...R