Ah, right, I was actually wondering if there were screws right up at the top end of the tubes, they're often black and barely visible.
It would seem that @murrayec could be right...
Another possibility would have been screws hidden by the bogies, but that's excluded by his pictures, too.
Sometimes, a bit of gentle pulling about can reveal where the splits are and you may still be able to part them - but there is always an element of risk.
Lima did, at times, mould the roof and windows in one clear piece, then painted the roof section.