Absolute piece of cake to get from Brussels to DLRP. I'd guess a 90 min - 2hour trip. There may even be a train that goes directly to the resort from Brussels as Eurostar stops at the park (Marne La Vallee). The train station is actually right outside the gates of the park. Maximum would be one change at Paris. Check out
Eurostar : Tickets, Bookings, Timetables, fares and offers
Any info you want (Hotels (on or off property), park info, Paris info blah blah) then just ask in this thread or PM me.
TT
