Safe to take the train to 8,000 ft from sea level when have COPD on 2L/min continuous flow oxygen?

My brother and I are trying to figure out a safe way to move our Mom from Massachusetts to San Diego so that she can live near my brother and his family. She very much wants to live near by her son, daughter-in-law, and grandkids.

She is in her early 70s. She has COPD and is on 2L/min of continuous flow oxygen all the time at sea level. She uses a BiPAP overnight and for 2 hours while resting in the afternoon. She uses a nebulizer a few time a day to take medication. She had a stoke 25 years ago and has limited mobility. She uses a walker at home and a transport wheelchair with assistance outside the home. She experiences swelling in her feet, and elevates them while resting in the afternoon. She lives independently at home with some help for things like laundry and transportation.

We are considering using the train to move her across the country. In a train, she could have an accessible private room, be able to move around and lay down as needed, elevate her foot, and use her BiPAP for naps and sleeping overnight. She would of course be accompanied.

The train combination we are looking at is the Lake Shore Limited (Boston to Chicago), spending one night in Chicago, and then the Southwest Chief from Chicago to Los Angeles. The Southwest Chief goes to almost 8,000 ft elevation around the Colorado and New Mexico border, and it stays around 6,000-7,000 ft elevation for over 12 hours across New Mexico and Arizona.

She is currently on 2L/minute continuous flow oxygen when resting at sea level, and she would take a portable oxygen concentrator which can go up to 3L/minute continuous flow oxygen. We are looking at the O2 Concepts OxLife Independence.

We talked to her pulmonologist about flying and taking the train. He said that flying would have higher risk because of the sudden pressure change, and favored the train due to the elevation change being more gradual as compared to an airplane. He did not see an issue with the total elevation, and said she may not even need to increase her oxygen above 2L/minute.

Living at 5,000 ft myself and visiting 9,000 ft on occasion, I know that I as a healthy adult feel the effects of altitude at 9,000 ft especially when I'm active, so I am hoping to get another opinion on this. I don't want to make things more difficult for my Mom than they need to be, but I just want to make sure this train plan is safe as the doctor said it is.

Is there someone than can tell me if they have done something similar or has experience advising patients on something similar?

For reference, this the full list of elevations of the various train stops on the 2 day trip from Chicago to LA. You can see it goes from 4,000 ft to 7,500 ft in just 3 hours, and mostly stays above 6,000 ft for over 12 hours.