Twilight has been busy as of late with her recent duties as a Princess. Which, to be fair, she doesn't know entirely what they are, so she has to prepare for everything. As she shares a lunch with Rarity going over all of her recent work, she mentions that she'd like to take the train up to the Crystal Kingdom for a formal diplomatic visit instead of a royal coach since all that time alone with the guards near her with nowhere to escape to would freak her out. Rarity immediately suggests bringing Tom the Rock along as luggage at the last minute so the train is only method of travel they can arrange on such short notice to accommodate him. Also, Rarity is now going with Twilight on her trip.

It's all perfectly sensible. Really.

*Written to take place near the beginning of Season 4.