The Postal Transportation Service (PTS) was the renamed successor to the Railway Mail Service of the United States Post Office Department from 1 October 1949. Although this branch of the service had been in charge of all transit mail, some parts had little to do with railroads, even though they were still the most important part of the service. In 1950, of the 32,000 clerks assigned to the PTS, only about 16,000 actually worked on trains. The remainder were in terminals, transfer offices, Air Mail Facility, Highway Post Offices (HPO), administrative offices, etc. Boat Railway Post Office (Boat RPO), Streetcar Railway Post Offices, and the Seapost Service had already been discontinued. The name of the Chief Clerk's office was changed to District Superintendent's office. During the preceding decades, only one or two clerks per year had lost their lives in wrecks and several years saw no fatalities. When Pennsylvania Railroad's crack "Red Arrow," New York City, New York & Pittsburgh, Pennsylvania Railway Post Office (RPO) Train 68, derailed in 1947 at Bennington Curve (West of Altoona, Pennsylvania), killing six clerks and badly injuring others, the whole country was shocked. Even with all the trains that had been discontinued, the several round trips of RPO service on trunk lines, along with the expanded service and the star route highway services connecting the RPOs, maintained very good mail service through the 1950s. In the 1950s the Post Office Department turned the supervision of what had been the PTS Terminals over to the postmasters where the terminals were located. Discontinuance of the mobile units The HPO and RPO routes were collectively referred to as mobile units. The next move by the Post Office Department, in 1960, was to put each PTS District Superintendent's office under a postmaster, calling it the Mobile Unit Section, c/o Postmaster. This put all RPO clerks under postmasters. When there was no longer a surplus of RPO clerks from discontinued lines to fill vacancies on lines still operating, both subs and regulars from the post office roster were used. This eliminated the need for PTS civil service examinations. In 1963, the Sectional Center concept of transit mail service was announced, along with the introduction of a ZIP Code on mail to facilitate mechanized processing. The ZIP Code made it possible to distribute all mail by numbers with machines holding the routing associated for each address. This was a far