HPO vehicles are an interesting area of study but an understanding of the state of transportation if the era is beneficial. By the late 1930s, roads and support facilities had grown beyond city and urban boundaries. Paved roads and highways were being connected throughout America and this began the eroding the existing monopoly on railroad freight and cargo transportation. Unlike the highly structured railroad tracks, roads could be connected literality door-to-door.
Following on the heels of the successful railroad mail cars, the Post Office had received a number of proposals for a ‘highway mail cars’ including one from Twin Coach in the late 1920s. But it was not until the late 1930s they felt the concept was feasible and began in earnest to implement the idea. In 1941 the post office implemented a pilot program which included three routes; they used this opportunity to test several different vehicles.
The following provides an overview of some of the Highway Post Office vehicles. Click on images below for slide show and more information.
The White Models 788/798/1100 buses were between 33 and 35 feet in length; they typically carried two postal clerks and a driver. The interior contained a sorting table, pigeon holes cabinets, and mail bag racks towards the front. The rear section contained a small bathroom and a general storage area with a capacity of 150 mail bags. There was also an external mail slot for accepting letters from postal patrons. The White 788 was powered by a 12 cylinder, 210 horsepower engine and even through the Model 788 was predominately made from aluminum, this bus typically got around 5 miles to the gallon.
White was most widely use HPO vehicle manufacturer with a total of 48 purchased between 1941 and 1949.
The second inaugural route, between South Bend, Peru, and Indianapolis Indiana was started on May 3, 1941 and utilized an International Harvester DR-700 cab-over-engine tractor mated to a customized trailer. Powered by a 400 cubic inch 6 cylinder engine, the cab-over-engine design did not support communications between the postal clerks and the driver. While it logged some significant mechanical issues, postal officials were also concerned with the driving characteristics of this rig. Given the need to navigate loading docks and other tight corners, jack knifing was considered an issue with this particular tractor-trailer combination. Because of all of these reasons the post office dropped consideration of a tractor-trailer combination and concentrated on the bus configurations.
There was only one of the International Harvester DR-700 vehicles purchased in 1941.
For the third vehicle in the inaugural pilot program, the Post Office purchased the 1941 Mack CM3G shown at left. This rear engine model proved less than satisfactory since it was designed for short city type routes. Once it was on the open road, it proved to be under powered for hilly terrain. Additionally it demonstrated less than ideal gas consumption. In 1948 Mack sold 15 Model C45GT to the Post Office and Post Office contractors. These diesel buses had more power and were better suited for open road travel.
The second most popular HPO vehicle manufacturer, the Post Office purchased a total of 18 Mack vehicles.
By 1953, White was exiting the bus business and the Highway Post Office program needed to find new sources. HPO contractors such as Gulf Transport Company of Mobile, Alabama purchased and implemented 1952 GMC Model TDM 4509s as shown at left. These 35 foot were built by GMC from 1949 to 1955 and they featured a 6-71 diesel engine with a manual transmission. It is unknown as to whether GMC or Gulf themselves customized these units but it is interesting that the records show that GMC delivered the units the units to Gulf in November 1951 but the buses were not put into service until February 1952.
There was a total of seven GMC 4509 used by Gulf in the HPO program.
Twin Coach was a vehicle manufacturing company based in Kent, Ohio from 1927 to 1955. In 1955, the bus manufacturing operations were sold to Flxible which was also based in Ohio (see below). They were pioneers in articulated vehicles and a number of these were used in the HPO program starting in 1951.