I have a several-years-old Garmin 60C, which I've used on several train trips.
Also as a Mac user, the general lack of support has been frustrating but not a complete obstacle. There are plenty of programs, and recently some from Garmin, that let you transfer waypoints and tracks and routes between a Mac and GPS. The sticking point is the maps: the unit can only hold 50MB of maps at a time, so every time you change locales you need to reload the relevant maps. Garmin
finally has a Mac program to upload the maps, but you first need to use a PC to convert to the Mac-compatible format.
One thing I've found very useful for this unit--it might not be an issue with the newer "high sensitivity" models--is to have an external antenna. Mine came with a suction cup base that I stick to the window of a train; it has a long cord so the unit can be passed to an aisle-sitting seatmate without losing GPS reception. Otherwise I have to hold the unit right up to the window to get reception. I haven't had any problems using the antenna on the train--nobody told me I couldn't suction-cup it to the window.
Another thing I found somewhere on the internet, years ago, is a list of Amtrak stations with latitude and longitude. With a text editor or simple script I could convert this into a list of waypoints, and then string all the station waypoints of an Amtrak route together. One of the cool things that I think any GPS unit can do is to compute not only your speed, but the rate at which you're closing in on a waypoint (which might be slower if you're not headed directly for it) and then give an estimated time of arrival. It's also nice when you wake up on a overnight train and can't see the mile markers and wonder where the train is.