If you are worried about mask compliance, you can be glad you are headed for East Glacier rather than West, and you can be assured the people on the train will take it more seriously than the rest of western Montana does.
The Blackfeet Reservation (East Glacier) is taking the virus seriously with much mask-wearing and travel restrictions still in place --- be aware that the east side of Glacier Park is not open yet, because the Reservation has refused to allow tourist traffic until at least June 30th, and there is no guarantee that will change on June 30th. (You don't say if you are planning on a tour into the park.)
Meanwhile, elsewhere in western MT... there are people wearing masks in generally-blue Whitefish... but in Libby and Columbia Falls, very few people wear masks, and trying to wear one results in verbal abuse from passersby, who think the virus is a hoax and only evil communists wear masks.
(For non-locals who wander into the thread: Montana had relatively few cases of the virus even in April, and Flathead County hasn't had a new case in 60 days - so there is a very widespread belief here that it is not a big deal. The reservation, and the town of Shelby just to the east where there was a bad outbreak in an assisted living home, are among the few places in the state where a majority of people are paying attention to the possibility of the virus coming back.)