UK railway stations no longer sell tickets for international travel (other than by Eurostar). If you ask at a railway station, they'll direct you to their official partner company, Rail Europe:
http://www.raileurope.co.uk/inter-rail/
At least, that's the official policy. Many ticket office staff will tell customers what they really need to know (so that they can avoid the booking fees which Rail Europe charge). The best place to buy tickets and passes for all European rail travel is from the UK offices of Deutsche Bahn:
http://www.bahn.co.uk/db_uk/view/products/inte rrail.shtml
If you'd prefer to buy over the phone, Deutsche Bahn will sell you tickets and passes on 08718 80 80 66.
Chris
PS: Deutsche Bahn also has the only integrated online European rail timetable. It will allow you to plan a journey from Middlesbrough to Moscow, or from Amsterdam to Athens, in just a few clicks:
http://reiseauskunft.bahn.de/bin/query.exe/en