Like most airports, you mean?
I'm not against subsidizing train travel, but you should be getting something for it. You could decide to add a train between any two places and dump money into it until it's cheap enough to fill up. The question is if you're causing people to take the train who would have otherwise driven/flown, or if you're just subsidizing a new market.
I don't live in either city, but the concept of taking one of the other long-haul Amtrak lines from Chicago to Seattle has seemed interesting to me. It's not that expensive and it would be quite scenic. That said, if I took it, I'd be flying to Chicago, then taking the train, and then flying back home from Seattle. That's not replacing a flight/drive, it's creating net new flights. Is that the sort of train line that we should be subsiding the existence of? I can see the value of doing it as a matter of culture and history, but if the goal is to reduce the carbon emissions of flying and driving, it may actually be counterproductive.
Maybe they think that trains are like planes, and that you have to stay on them for the entire Chicago-LA trip, rather than getting off (or on) at one of the 50 other stops?
Not sure what you mean by this. There is already a passenger route between Chicago and LA. Has been for a long long time. It's called the Southwest Chief it runs from Union Station LA to Union Station Chicago multiple times a day.