It seems like they don't allow Andermatt - Chur passengers: you would block seats for people doing the full Zermatt - St. Moritz journey.

Looking at leaving Zermatt or Brig the first train has 10+ seats left, second departure only 4. Most tour groups book the train months ahead.

However it's not an issue: use the hourly reservation-free regional trains along the same route. You'll save money, have more space and could even open windows on the second train!

The short connection in Disentis is guaranteed, don't worry about that.