Can someone tell me whether I can order a travel card for my Oyster Card
on
the Internet and have it activated from a train station (i.e. not a London
Underground station).

There may be the odd exception, but basically no you can't. You have to
nominate a tube station and it will be loaded next time you use the gates or
validators at that station. Fine for renewals of travelcards for those
using the tube but not helpful for getting the first travelcard onto the
card or for those who only want bus passes or Pre-Pay and don't use their
card for tube journeys. Basically means you have to purchase your
travelcard at a ticket office, ticket machine or at a 'Ticket Stop'
newsagent.