To sit in the “Angel’s Seat” is to sit in the highest place in the train. The cupola of the caboose.
The first “Angel Seat” was nothing but a chair on a stack of boxes with your head sticking out of a hole in the roof. Now you know why cabeese (think goose/geese) are called crummies and hacks
Nobody knows where the term cupola or caboose actually originate from, much less the term Angel Seat. Though many people have speculated, some even credibly.