Going from London to Luxembourg. Getting the Eurostar ticket to Brussels is no problem. However, is there a way to get the Brussels to Luxembourg ticket prior to my arrival in Brussels so I can go directly from one train to another without having to bother with a ticket window or machine. I went to the SNCB site where I can buy a ticket but didn't get as far as delivery options because I didn't want to log in. (And please, no RailEurope suggestions.)
According to this brochure (in French) on the SNCB site, if you order tickets online or over the phone, you will have to present a dossier number and a retrieval code at a station window. Your ticket will then be printed for you. However, it says that under certain conditions you will be able to print the ticket yourself. I imagine that the only way to find out if self-printing will work for your particular route is to register on the site and then at least do a dummy booking.
I can print it out at home...in fact, I just realized that the Eurostar ticket is actually good to any train station in Belgium. The last station in Belgium, before Luxembourg, is Arlon. So, I can save money by only having to buy a separate ticket from Arlon to Luxembourg.
Well done!