Following the wisdom on this forum, I'm going to book a 9 a.m. entry to Versailles. I plan to allow enough time to get there by 9, of course. But I'm wondering--in case there is a train delay or something--how much leeway is allowed for entry after 9. The next time is 10, so if you get there before 10 with a 9:00 ticket can you still get in? Before 9:30?
Also, can you buy RER tickets prior to your day of travel? I'm thinking to have the tickets already in hand before that morning.