Hello, I would like to go from Disneyland Paris and visit the Palace De Versailles (and Return the same day), and do I have to book in advance? Also what is the cost? Thank you
The quickest route from Disneyland Paris to Versailles would be the RER A train from Disney till La Defense, then switching to Transilien L train towards Versailles Rive-Droite, taking that train to its terminus. There are many different trains passing through La Defense so you'll have to check overhead displays to find which platform a "Versailles Rive-Droite" train will stop at. Transilien L line trains depart from St. Lazare station and only make a quick stop at La Defense on their way towards Versailles.
No booking in advance is possible. The seats on the RER A and Transilien L trains are non-reserved. You can buy tickets at any time though (no expiry dates on these tickets).
Disneyland to Versailles trains will cost 11.45€ one-way for adults and 5.70€ for kids 10 and under. You can buy return tickets at the start of the trip. It will just be another set of the same tickets you will use to get back.