We’re visiting Paris for the first time ever in February and we’ve booked accommodation in the Ville-D’Avray area. We’re going to use one day and travel to Disneyland as a birthday treat but we’re hoping to keep our travel costs as low as possible by getting the train. I was wondering if you could suggest the easiest route to take please?
The train route would be to start at Sevres-Ville d'Avray train station, taking either a Transilien L train (direction Paris Saint-Lazare) or Transilien U train (direction La Défense Grande Arche) until station La Défense. At La Défense station you would get off the train and transfer (using same ticket) to RER A train to Disneyland (direction Marne la Vallée) and ride to the end of the line, which ends at Disneyland.
The cost for a single-use RER ticket, one way, would be 10.65€ so it's actually cheaper to use a Mobilis day ticket, 1-5 zone which costs 17.80€ and can be used in both directions, going to Disney and coming back to Sevres-Ville d'Avray station.
Keep in mind the Mobilis day ticket will expire at midnight so you need to complete your journey back (i.e. arrive and exit Sevres-Ville d'Avray station) before midnight if you go this route. Otherwise, use RER tickets in both directions and you can return whenever you like and you're only paying about 3€ more.
If you go with RER tickets, it's easiest to have the ticket window agent create you the ticket, which needs to specify your start and end stations. Otherwise you can use the ticket vending machine and slowly type in the destination station (Marne la Vallée-Chessy). You can buy Mobilis day tickets from ticket vending machines as well.
If the train strikes are still ongoing in February, be sure to check the Paris 2020 train strike page for the Disneyland train schedule in case the first/last train times have changed.
Have a good Disney birthday visit.