There's no reason why you shouldn't travel by train between places a great distance apart, it just needs to offer a benefit when compared to flying or driving. If you've got the option between spending two hours in an airport waiting for the TSA agent to poke and prod at everything in sight, spending another eight hours on the plane crammed into a tiny space, and then have to wait for your bag to finally show up on the conveyor, maybe it is worth taking longer to sit on a train, not have to deal with the TSA, have a space big enough that you can stretch your legs a bit, read, maybe do some work or take a nap, and then carry your bag off with you rather than waiting for it to show up while hoping it didn't find its way to Bangkok or something.
The eternal question is what it offers when compared to other solutions. If it costs more and is less convenient people won't use it.
I often think about transport when it comes to a situation like visiting friends of ours. It takes about 10 hours to drive from our house to theirs. We could fly it, but it would be 30 minutes to the airport, an hour or so for check-in and security, probably 2-3 hours in the air assuming no delays, then an hour to drive from the airport to their house. And we'd need to rent a car, or we'd be totally dependent on them driving us everywhere. So we'd save about five hours flying compared to driving, but we'd be limited in what we could take and we'd be subjected to changing rules in airports, the tiny seats on planes, delays, and extra costs every which way.