Overall, there are several GPS receivers with different power and computer connection options - as indicated above and in previous threads posted over the years. Here are the configurations I've used:
Garmin e-Trex Vista (the older model, not the HC model) with moderate success. I use a 120VAC to 12VDC converter for power and RS-232 interface to WIN XP computer running DeLorme Street Atlas for display. I really don't need the computer to use the e-Trex, but the computer provides nicer display.
Garmin GPS III+ with antenna extension cable w/suction cup attachment. As with the e-Trex, I use a 120VAC to 12VDC converter for power and RS-232 interface to WIN XP computer running DeLorme Street Atlas for display. The antenna extension cable enables you to move the GPS receiver to an out of the way location.
Garmin Nuvi with window suction cup mount. I use 120VAC to 12VDC converter for power. I use in off road mode, with volume muted. The Nuvi receiver is much nicer than the receiver on the e-Trex or GPS III+. So, you should have reasonable reception on the train. I transfer the data to my computer after the trip.
I haven't tried the DeLorme Earthmate USB receiver, but would expect reasonable performance from what I have gathered from other GPS related forum discussions. One benefit is it is powered by your computer. Realizing you have a MAC, I would try WINE or other windows simulator.
Hope this helps!