The final episode has been revealed.
Episode 24 - April 7th
Title: MMMystery on the Friendship Express
Description: A cake Pinkie Pie is guarding for a contest is ruined on the way to Canterlot and an investigation begins.
I'm a bit disappointed, I was hoping for Scootaloo or Celestia to have gotten one episode this season. Oh well, there is always Season Three.
This could potentially be centered around the main six, the description only tells us that Pinkie was guarding a cake.