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.