Tom Ferguson worked for the United States Postal Service. He was not your everyday mailman, for he worked in a moving railway postal car. The line he worked on was the Missouri-Kansas-Texas (KATY) railroad. The concept of the railway postal office (RPO) was not new. The first railway post office began operating between Chicago and Clinton, Iowa on the Chicago and North Western Rail line. By 1869 the Railway Mail Service had expanded to about every mail railroad line in the United States. 
Tom and his wife Lola moved with their young family to Sedalia, Missouri in the early 1900s. While there, he secured a job working as a mail clerk on the KATY line between Sedalia and Parsons, Kansas. If one were to travel between the two locations by car today, it would take three hours and twenty-eight minutes. It would probably take much less time by train. 
In June of 1911, the family moved to Webster Groves, Missouri. Tom continued to work for the U.S. Postal Service on the rail line between St. Louis and Sedalia. If one were to travel between the two locations, it would take two hours and fifty-one minutes, a little less than the trip between Sedalia and Parsons. 
As a railway postal clerk, Tom would have loaded the mail onto the car, sorted, canceled, and filled sacks with mail for delivery to post offices in cities and towns surrounding the rail line.
The postal rail car was behind the locomotive. Sparks belched from its stack. The lighter wooden mail cars, sandwiched between the heavier locomotive and passenger cars, provided many opportunities for the mail clerks to suffer severe injury from a wreck, the sudden lurch of the train, or an errant spark that could set the postal rail car ablaze. 
According to trains.com, “RPO clerks were a special breed. Although they were Post Office employees, they were as much railroaders as postal workers.
A century ago, when both vocations held wide appeal (and when railroading was hazardous and mail shipments subject to robbery), they were heroic figures. An RPO clerk had to be familiar with thousands of towns and instantly know the best mail routings to them. On tests, 97% was the passing score; on the job, accuracy and honesty were well over 99%.” 
This job provided the Ferguson family of seven with a solid middle-class income that took them safely through the Great Depression of the 1930s.
The RPO system was still vibrant in the early 1950s. However, by the 1960s, the number of passengers on trains began to decline. Seeing no future for the passenger, the United Postal Service severely cut their RPO routes and eventually ended in 1977. 
Tom kept this job until the early 1940s when he began working as a salesman for a local St. Louis real estate firm. He was in his late fifties. The new occupation was probably not as exciting or physically demanding as his days on the Katy line but certainly safer.
 Wikipedia, https://en.wikipedia.org/wiki/Railway_Mail_Service