Hi emjs
Thank you for your request.
The Swiss Travel Pass is valid from Châtelard Frontière station, you have to buy a ticket from Chamonix Mont Blanc to Châtelard Frontière. The Swiss Travel Pass is also valid to Zermatt.
You can find more information here (Map of Validity).
For your trip between Chamonix and Zermatt, it is not necessary to reserve seats. For more information, please contact our Railservice on +41 848 44 66 88 (CHF0.08/min.). If this number does not work, please try +41 51 225 78 00.
I wish you a nice weekend.
Best regards
NicolasK