can i use my navigo easy card on the RER train - it didn't work at the RER train station this evening
You can only use a Navigo Easy card for taking RER trains within central Paris / Zone 1, not beyond.
Within Zone 1/central Paris, RER trains can use the same fare as a Paris Metro ticket (officially known as a Ticket t+).
Beyond Zone 1 RER trains require RER tickets (officially known as Billet Origin-Destination), but Navigo Easy cards cannot hold RER tickets, only Metro/Ticket t+ tickets (and a few other fares such as airport buses, day passes).
So if you were to take a RER train from say Chatelet les Halles to Auber, yes, you should be able to use your Navigo Easy.
If you were to try to take a RER A from Chatelet les Halles to Marne la Vallee, you can enter the fare gates at Chatelet, but upon arrival at Marne la Vallee, your Navigo Easy would not let you exit through the fare gates. And at any point along the RER A past Nation, you would be fined by controllers for not having the correct fare, if caught.
If you were to start your journey outside of Zone 1, I would assume your Navigo Easy would not let you past the fare gates, since a Billet Origin-Destination is required. (I have not tried this myself.)