Returning to the magic train?

[Rizzice]

So I finished a bit into the game already.. and then I realized I was missing a treasure from Twilight Town. I remember that there was a treasure chest I missed back at Mickey's teacher's tower (forgot his name).. the one I took the magical train to get there with.. is there any way to get back there?

I went back to the station plaza, but the train isn't there....

Maybe I will go there later in the story again? Help please! D:

[Ouch!]

It becomes available again later. I'm not quite sure when exactly it is there, but towards the end it's back. The tower is crawling with Nobodies, though.

[Giga Guess]

At the end it's available for sure.

[Skyblade]

Once you find the second Twilight Town and the World that Never Was, it opens again.