Amtrak used to have a train (The Pioneer, IIRC) that ran from Portland, OR to Salt Lake City, UT via Boise, ID. When they killed that train a few years ago, they put an Amtrak “Thruway” bus to cover part of this route. This bus apparently stops in Nampa, Boise, and Mountain Home on I-84, hence the Amtrak listings for these as “stations”. These are comfortable buses, but not trains, and the stops will probably look like regular bus stops but with an Amtrak logo (i.e. they’re not manned stations; you buy your ticket on the bus).
So, you can get from Mountain Home (MTM in Amtrak-speak) to Salt Lake City (SLC) on an Amtrak bus, which will get you onto a train without having to go to Sandpoint or Spokane (which are, to put it mildly, a little further North than you probably wanted!).
Amtrak used to have a train that ran from Salt Lake City to LA via Las Vegas and Palm Springs, but they canceled it a few years ago (sound familiar?). So, you could have gone from Mountain Home to Palm Springs with one change of train until recently, but not any more. If you punch “MTM” and “SAC” (i.e. Sacramento) into the Amtrak trip planner, you’ll see that you can leave MTM at 12:45pm on Day 1, and arrive at SLC at 6:45pm. Then take train #5 at 11:59pm from SLC to SAC, arriving at 2:15pm on Day 2.
Now punch “SAC” and “BFD” (i.e. Bakersfield) into the trip planner, and you’ll see a train (#704, the San Joaquin) leaving Sacramento at 4:25pm and arriving at Bakersfield at 9:43pm.
Now comes the fun part: Amtrak trains don’t run south of Bakersfield, so you’re back on a Thruway bus again. If you plug “BFD” and “PSP” (the Palm Springs Amtrak bus stop) into the trip planner, you’'ll see that the only bus leaves BFD at 1:55pm and arrives at 7:05pm. So, you’d be stuck for over 16 hours in Bakersfield. Bwahahaha!
Alternatively, you could take the bus that leaves BFD at 9:50pm (it’ll wait for the incoming train) and arrives in Los Angeles at 12:35am (i.e. about 36 hours after leaving Mountain Home). There’s a train (the Texas Eagle) that goes from LA to Palm Springs, but it only runs three days per week. To be honest, you’d be better off taking Greyhound or some other service at this point (or perhaps getting a friend or relative to pick you up in LA?).
So:
- Yes, the Amtrak online reservation system sucks, and often responds in the same manner as the country bumpkin in the old joke, who when asked by the tourist how to get to a certain place responds “You can’t get there from here”.
- You can do it end-to-end via Amtrak, but I wouldn’t recommend it. You’ll either have to overnight in Bakersfield or LA , or you could take a Greyhound for the last leg from either of these to Bakersfield.
- The above routing gives you a 2 hour 10 minute wait in Sacramento. This may sound fine, but less so if you bear in mind that the train from SLC to SAC can be several hours late, since it comes all the way from Chicago, so you’d be stranded in Sacramento.
In summary, it’s possible to do it, if you (as Civil Guy suggested) split your supplications to the Amtrak computer into several segments, but all in all I think the computer’s doing you a favor in not giving you a route. You could do it, but you’d end up asking yourself why you’d bothered.
Which is a shame, because riding trains in the Great American West can be an amazing experience. You see parts of the landscape that are completely missed by someone driving on a freeway.
I hope this answered your specific questions about the process, even if it doesn’t provide you with a satisfactory solution!