Doughnut Joe's shop is already in Canterlot; why would he need to take a train there from Ponyville?
His reason for being in Ponyville is asked for and gotten when he shows up on the train — he was there to pick up sprinkles (presumably some kind of special sprinkles that are easier to find in Ponyville than in Canterlot) for his dish. That just leaves the question of why he took his donuts with him to Ponyville.
Maybe the sprinkles needed to settle overnight?
If Applejack was able to resist the temptation of stealing the cake because she's the Element of Honesty, shouldn't that apply to the other Elements as well? Like the Element of Loyalty wouldn't betray a friend's trust like that, the Element of Generosity wouldn't be so greedy, or the Element of Kindness wouldn't screw over her friend and an overworked pair of new parents. That is not to say that the characters have to always have to act according to the Element, but to assign such behavior to their Elemental status when the others apparently fail in theirs just seems jarring.
I think a lot of that stems from the fact that Applejack is usually the Straight Man who acts as the voice of reason. Everyone has weird quirks, but Applejack's aren't played up nearly as often as the other cast members. Therefore, people assume that Applejack is more committed to her virtue because she doesn't have (quite as many) silly antics to get in the way.
Yeah, but the problem is that people assign her victory to her status as the Element of Honesty alone, as opposed to her tendency to be more committed to her virtue.
It's a Plot Hole within the fandom, since half of it likes to tie everything to the Elements of Harmony like they're the most defining traits ever. Simply put, the assumption of her resisting temptation because she's the Element of Honesty is just a fan opinion, and oddly, makes the least amount of sense- it's not dishonest to take a bite of the cake, as long as she fessed up to it (which she does in a deleted scene). It has NOTHING to do with the Elements or commitment to them. AJ just has more self-control than the others do, and it's possible the reason she wanted to hit the hay so early was because she was so tempted and figured getting the cake out of her sight would help her get it out of mind until the next day.
Alternatively you can just assume the deleted scene where she did confess to taking a bite happened offscreen. It was cut due to lack of time after all. Therefore everyone was Not So Above It All concerning their Elements.
This episode also highlights the best possible reason to hire Pinkie at Sugarcube Corner: Her loving description of food can make anypony hungry and eager to buy more from them!
Why did Big Macintosh have a hard time hauling the MMMM? He was trying to keep it from falling over!
