15 hours is doable regardless, as you can sleep half of it and if you have a tablet and battery pack, watch away any idle boredom, though going through new terrain, the view should suffice. Your big decision will be that 26 hour ride or not. You could always stop somewhere in between if that seems too daunting, like in Chongqing or Chengdu.

I'm a big fan of travel writer Paul Theroux, who's "done China" twice by train. Think I'll do it.

If possible, take a train that starts from your departure city, otherwise, you could end up with some smelly bedding for 15 or 26 hours.

LOL. Sounds like advice from a fortune cookie. thank you.