You can book this from 3 months before your Glacier Express trip. Here are the links to book through Rail Europe World:
Other points of sale are listed here.
All you need to know about traveling by train, bus or car in Switzerland.
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Swiss Travel Pass promotion: 30% discountAvailable until 31 October 2022. Book 6 months ahead.https://shop.switzerlandtravelcentre.com/swisstravelpass