These three post cards are pictures of people interacting with what looks like a car from a passenger train. In reality, they are all picture backgrounds and foregrounds to give the illusion that the person is at the train. They are souvenirs of their visit to Denver, Seattle and an unknown city in the form of a post card that they can mail to their loved ones back home.
The top left one is the brainchild of one H. Schwartz of Denver, Colorado. The sign says that it was taken at his Lakeside Park studio in Denver. He also had another studio at 320 17th Street in Denver.
The top right one refers to a train that goes from Seattle to Spokane in Washington State: the North Coast Limited. The North Coast Limited was a named passenger train started by the Northern Pacific Railway between Chicago and Seattle. It began operations on April 29, 1900 and made its final run as a Burlington Northern Railroad train on May 1, 1971.
The top post card, in the scan here to the lower right, is the back of the Schwartz card. On the left side of the card is the two addresses of his studios. In the corner where the stamp is placed it tells us that the paper used for the picture is of the PMO brand. PMO was used between 1907 and 1915. All four triangles in the corners are pointing up. I am not sure if this is similar to AZO paper symbols, but if it is, that means this card was printed between 1904 and 1918. If there is no correlation it is from between 1907 and 1915.
PMO belongs to the category of Chloride Prints - These silver chloride papers in a gelatin emulsion were much faster than traditional papers that required sunlight exposure. They became known as gaslight papers because of their ability to be exposed indoors under gaslight.
The third post card is printed on AZO paper. Again, from the Metropolitan Post Card Club of New York City website: Gelatin Prints – “These chloride papers were made with very small particles of silver suspended in a gelatin emulsion. They tend to be vulnerable to contamination and can easily deteriorate. They were much faster than collodion based papers and were able to be exposed indoors under gaslight lamps (gaslight paper), but they remained slow enough to be used only in contact printing. Its ease of use made it the most popular paper on the market for real photo postcards. They were manufactured with postcard backs.” This post card’s triangles tell us it was printed between 19048 and 1918.