These postcards used just two colors of ink–dark green and a reddish orange–so I’m calling them duotones. NP published an extensive series of duotones that overlapped the “air-conditioned” and “completely air-conditioned” eras. Today I’ll show marked “air-conditioned,” meaning they were probably issued in 1935 or 1936.
The first one shows a standard photo of Old Faithful, except that there appear to be just six people watching this eruption. Today, an average of about 2,000 people watch each eruption during summer months.
The second postcard shows the Tooth Rock Viaduct on the Columbia River Highway, one of the more spectacular views on that road (unlike the view in a previous NP postcard). This viaduct still exists but current is open to hikers and bicyclists only.
The third postcard shows the Taylor Peaks, which are located in the Lee Metcalf Wilderness south of Bozeman up Gallatin Canyon and just west of Yellowstone Park. The highest peak of the Taylor Peaks is called Koch Peak. This is another sight that very few NP passengers would ever see in real life.
Finally, here is the only duotone postcard I’ve seen in landscape format rather than portrait view. Unfortunately, the site I found this on didn’t have the back of the card so I can only show the front. I suspect the photo was taken in the North Dakota Badlands.