I would agree with @Ron045 to paint the white parts before glueing to better hide your crack.
I would also prop something (scrap wood, etc.) up next to the roof to support it while the glue thoroughly dries.
Once that dries, as someone mentioned, put something underneath. I had a steam locomotive cab that needed support and my friend and I used a piece of a tuna can to support the roof. That might be a little overkill for your caboose, but just thought I would mention it.
Good luck and please post your finished project pictures.
Tom