The 1857 train from Schiphol to Duivendrecht departs from Platform 2. The one at 1940 departs from Platform 1. Onward connection to Cologne in both cases is from Platform 8. (Where would be without www.bahn.de??)

What's more, you can buy your ticket online at www.bahn.de - but will need to have it posted to you as it's a traditional paper ticket (they do print-your-own for some routes but not this one). Tickets are not tied to specific trains so if you miss the direct train you can use the ticket on the later connection.