The train to Disneyland® Paris from London is a Eurostar high speed train with travel time of 2 hours 55 minutes. Tickets start at £69 return.
The train to Disneylqnd Paris leaves once a day at 08:35 from London St. Pancras International Train station (King’s Cross Tube station) and arrives at Disneyland Paris park gates at 12:30 (Paris time, which is one hour ahead of London time).
More information: Eurostar train to Disneyland Paris.
If you plan on visiting Paris before making a trip out to Disneyland Paris, there is also city train service to Paris Disneyland. The train to Disneyland from Paris is the RER A commuter train which offers non-reserved seating train service between Paris city centre and Disneyland at Marne La Vallée–Chessy train station. Ticket price is one way (or twice the amount for return).
Schedules, maps, tickets/passes and more information can be found here: Paris RER to Disneyland.
Trains to Disneyland Paris also leave directly from CDG Airport arriving at Paris Disney park gates in less than 15 minutes with ticket prices starting at one way.
More information: TGV Trains from CDG to Disneyland Paris.