Apr 15, 2012 - 8:45 AM
I have checked and I came to the same conclusion!
Your Eurail Pass will give you free traveling on trains of Rhaetian Railway (RhB), i.e. from St. Moritz to Disentis with the Glacier Express. The Glacier Express route covers two different Railway Companies, the Rhaetian Railway and the Matterhorn Gotthard Railway (MGB). The MGB has a different policy concerning discounts with the Eurail Pass. You'll get a discount, but I'm not sure whether it will be 25 % or a 50 %.
As far as I know, you can make your reservations for the Glacier Express
independant from having your tickets. As it is not sure how your Eurail Pass will work with ordering your tickets for the leg Disentis to Zermatt, I suggest you ask the MGB through this link
In general, it would be wise to make reservations as early as possible: it would be a pity if you can't make reservations because there are no seats left (as you'll travel during high season)!
I hope this will help you and wish you a very pleasant stay!