What I think Will happen on MMMystery On The Friendship Express

1. Pinkie Pie is given the important job of guarding the royal cake but after she lets her guard down, she finds the cake partially eaten. She begins an investigation to find out who-dun-it.

2. Pinkie enlists the help of Twilight to help her solve the mystery. Pinkie and Twilight try out a range of different tactics to work it out.

3. Pinkie grows weary of questioning everypony and stressed about greeting the Princess with a half-eaten cake.

4. Twilight admits to eating the cake and apologises to Princess Celestia. Celestia tells Twilight not to worry.

5. Pinkie learns that sometimes the answer is right in front of you and that the best way to reveal the answer is to be approachable.

That's all I got. I did read a mention of Weird Al on EQD in relation to this episode. It would be amazing if he appears in this episode.