Hi - I need to use the Transilien Paris-Saint-Lazare Line L on Saturday and Sunday to go to and from La Defense. I'll be travelling from Gare du Nord. Do I need to purchase a special ticket to use the Transilien?
Transilien trains will use the same style of train tickets as RER trains. I call these tickets RER tickets as it's easier to remember / understand / less confusing but. To be more precise they are billets origine - destination or billet Ile-de-France. i.e. customized Ile-de-France train tickets which specify both the start and end stations on the face of the train ticket and valid for both RER and Transilien trains interchangeably.
So, you can buy a RER/Transilien train ticket from any of the ticket vending machines or ticket booths at Saint Lazare or Gare du Nord and create yourself a train ticket for Saint Lazare - La Défense.