Hi Tom,
Here is a sample route between Versailles & Chantilly.
RER C from Versailles Rive Gauche to St-Michel Notre Dame in Paris. (2,90€)
RER B from St-Michel Notre Dame (direction Aéroport CDG) till
Gare du Nord train station. (transfer included in ticket)
A Ter train from
Gare du Nord to Gare Chantilly Gouvieux (7,20€)
At
Gare du Nord you'll have to purchase a "Grandes Lignes" train ticket for Gare Chantilly Gouvieux. This can be done at the Billets Grandes Lignes at
Gare du Nord or via the Ter ticket vending machines if you have a smart chip PIN# protected credit card. The train you'll want to board will likely be listed as Gare de Creil (the terminus station) rather than Chantilly. Travel time is about 1 hour 30 minutes.
That may be a slightly complicated route, so here is another that's easier to execute.
RER C from Gare de Prochefontaine in Versailles in direction Gare de Versailles - Chantiers until Gare d'Austerlitz in Paris
RER D from Gare d'Austerlitz to Gare de Chantilly Gouvieux
This option will take nearly 2 hours for the trip. Price will be similar to that of the more complicated, but faster route above.
To find these routes I used
Transport Ile de France and
Transilien websites.
Although not related to this trip, my
Paris-Beauvais train article has some information on Ter trains that you might find informative.
Best regards,
Ben