Does USPS still use rail for mail? - Model Railroader Magazine

IIRC, RPO service ended in 1967. After that, the post office still used RRs to ship mail, but it's what used to be called storage mail (as opposed to that which was actively handled and sorted on RPOs.) This could be in a dedicated rail car, but like much other traffic, is now usually in a semi or container that is handled as intermodal traffic.

It also used to be the case that postal traffic was handled on passenger trains, because that was the fastest way to get between two points. Now, lots of high priority traffic means that the post office can get stuff where it needs expeditiously without needing to rely on our somewhat constrained passenger system.