Inaugurated at the same time as the Black Hawk and Aristocrat, the Ak-Sar-Ben (Nebraska spelled backwards) connected Chicago with Omaha and Lincoln. I don’t have any letterhead for the Ak-Sar-Ben, but here is stationery for the Ak-Sar-Ben Zephyr, which sort-of replaced the Ak-Sar-Ben.
Click image to download a PDF of this letterhead. Click here to download a PDF of the matching envelope.
According to Wikipedia, the Ak-Sar-Ben Zephyr began service in 1940 as an eastbound-only, daylight train. The train returned to Chicago under the name Advanced Flyer. I suspect the Ak-Sar-Ben remained on the timetable as an overnight train. In 1947, the Ak-Sar-Ben Zephyr was replaced by the Nebraska Zephyr, which made a round trip under the same name. At that time, the Ak-Sar-Ben remained on the timetable as an eastbound-only overnight train.
In 1953, the Ak-Sar-Ben Zephyr was restored to the timetable as an overnight train, replacing the Ak-Sar-Ben. The Ak-Sar-Ben Zephyr used vista-domes and other cars lettered for the California Zephyr, allowing the Burlington to utilize Cal Zephyr cars that would otherwise sit in Chicago yards. The Ak-Sar-Ben Zephyr was finally discontinued in 1970.