Here are five postcards from two different eras featuring the Denver & Rio Grande railroads. The first three were printed in Germany, which dates them to before World War I.
Though issued by the Colorado New Company, each card prominently notes that the featured scene is on the “D&RG RR,” suggesting that they were issued with the cooperation of the railroad. The railroad probably gave the Colorado New Company license to sell the cards in Rio Grande train stations and on board trains.
Where Castle Rock is on the Rio Grande’s route between Denver and Colorado Springs, the Grande River–now known as the Colorado River–is on the route from Pueblo to Grand Junction. Denver passengers taking the Rio Grande to Utah or California would pass both scenes, but passengers from the east would be most likely to join the Rio Grande in Pueblo from a Missouri Pacific passenger train, and so miss the Castle Rock scene.
The angle of the above photo makes it appear that Glenwood Canyon is extremely narrow, but in fact is in fact much broader, as suggested by the postcard below.
This card and the one below were both issued by the Rio Grande and have the “Scenic Line of the World” logo and name Rio Grande in speed letters on the back. The number on the card below, “7A-H837,” indicates that the card was issued in 1937. I suspect the card above is newer, but probably from before 1949, or it would have mentioned the California Zephyr.