What is an HD radio? Have I missed something, I didn't know we had that yet. I am still dealing with the loss of all TV reception in rural areas last year.
If you are referring to satellite radio, you will get recption if your window faces the satellite. When the train turns around curves you wll have dropouts.
A few years ago I experimented with GPS and computer wi-fi from Amtrak, and found that both work much better from the upper level of a superliner (if you have a choice) than downstairs. If you have an external antenna with a rubber suction cup to stick on the window, you will do OK. If your device sits in your lap, forget it except for the wi-fi in metropolitan areas.
If you are upstairs in a sightseer lounge car, you should also be OK.
Having windows between you and the bird(s) is always the key.