Years ago the U.S. Postal Service published a list called "The Postal Bulletin". This had information on all changes going on in the postal service and also included RPO service. These bulletins were scanned and are online. While looking for something else I came across this from May 1, 1940:
RAILROAD SERVICE
Discontinued
From Apr. 30.1940, service over that part of route 114706 of the Denver & Rio Grande Western R. R. Co., between Tacoma and Silverton, Colo., 21.42 miles, is discontinued.
Rich Wilkens