As is usually the case, if the problem gets bad enough people will find acceptable solutions. Low density branch lines will eventually be mainly run by battery powered locomotives. I said eventually. So much research is now being done on battery systems because the market has just exploded: Electric cars, electric bikes (check out the volume in China), power storage in homes and a real big one, utility scale power storage. Utilities in California are required to increase their grid storage so as to balance out the generation sources. Yet one more field in which California will become a leader.

A branch line operator electrifies part of their heaviest line and locos charge up while in motion. Then on the low density lines you run on battery. The problem with hydrogen is that no efficient way has yet been found to make it from water using electricity. It is better to just use the electricity directly or store in in batteries.

This is down the road apiece. As has been pointed out, there are lots of other ways to reduce CO2 emissions that are easier. The main problem right now is having to wait for enough bad things to happen to climate change deniers that they come around on their own. You can't convince them. They just get their backs up. "Me Wrong. Would the Koch brothers lie?"