Railway Post Office service on the Grand Trunk Western Railroad came to an end on May 27-28, 1970, with the termination of the Port Huron & Chicago RPO on trains 155 and 156, the International.
With the cessation of en route rail mail sorting and distribution between Port Huron, Mich., and Chicago, Ill., the once-vital communication link between Canada and the United States has been severed, leaving three major cities -- Toronto, Lansing, and Chicago -- without connecting overnight service.
Fifty years ago there were 55 RPO routes crisscrossing the state of Michigan. Today only one route continues in regular operation, the Buffalo & Chicago RPO, which runs between the Motor City and the Windy City. Paul Maximuke
Trains Magazine, 9/70 pg50