It is of interest to note that the mail car carried on the "Red Wing"
was the Newport & Springfield(MA) RPO route, a USPO operation so
typically assigned a B&M car. On the other hand, the mail car on
the "Alouette" was the Newport & Montreal RPO route (even though the
car ran through to WRJ in those years), a Canadian P.O. operation so
typically assigned a CPR car. That's not to say a Canadian mail car set
up for Canadian mail service (not to be confused with a Canadian mail
car set up for U.S. mail service, an entirely different animal) was
never used for a U.S. RPO route but it would typically not be done on a
regular basis as far as I know.
JSH
