What happens to Optimus' back end of the track when he transforms?

[Da_Joka]

It just magically appears, and disappears. I'm talking about the gray rectangle that stores items.

[Spider-Steve]

You answered your own question (i think):

It appears and disappears magically!!

It's like that question about Clark wearing boots/shoes and his Clark-Supes haircut.

Sometimes there are things that can't been explained.... It just happens!

Steve

[Avangarde]

It was linked to another dimention where it was pulled via a vortex and was stored there until otherwise. :whatever: