Traveling Post Offices. 
Traveling Post Offices are also known as railway post offices. This is a special railway car set up as a Post Office to pick up bulk mail at stations, sort it, and deliver it to appropriate connecting point for delivery. The first car entered service in1855. (Figure 1.) In 1865, the first TPO car ran between Dublin and Belfast. Starting in 1866, a car went from Dublin to Omagh, and was later extended to Derry. By 1910, thirteen cars were in operation. In 1916, a modern 8 wheel car was built and continued in use until 1967. (Figure 2.) These cars were equipped with special apparatus that can pick up mail from special trackside stands without the train slowing. Mail could also be delivered to stations without stopping. (Figure 3) The cars were normally attached to express passenger trains, and followed a set schedule.
Traveling Post Offices have been phased out due to decreased train schedules and routes. These cars were equipped with special cancels that identified the route used. Example shown is Galway-Dublin. . (Figure 4.)
A preserved example of one of these cars can be seen at the Railway Preservation Society of Ireland museum and operating center at Downpatrick. The car can also be seen in operation on RPSI tour excursions.