Preserved Railways Letter Stamps. 
Starting in the 1964 as railways were converting from steam to diesel engines, a group of hobbyists started the Railway Preservation Society of Ireland (RPSI). This 32 county group was based in the of railway shops at Whitehead, Co. Antrim, Northern Ireland. This was originally the shops of the London, Midland and Scottish Railway, Northern Counties Commission. With the cooperation of the Northern Ireland Railways (NIR) and the Coros Iompair Erieann (CIE)(National railway of the Republic of Ireland) the group has access to all railway routes, and runs several excursions each year. The railway was authorized to originate and transfer mail to other railways and the Post Office under the provisions of an 1891 Act of Parliament that was still valid.
Since the RPSI conducted steam train excursions to various parts of Ireland, in particular to locations where passenger trains no longer operated, they decided to create railway letter stamps and sell souvenir covers to support their excursions. The first stamps were issued in 1980 and proved quite popular. (Figure 1 ). These stamps were used on tours, each with a special cachet. (Figure 2). A second issued followed, and was sought by collectors (Figure 3).
A slight problem arose since the trains were operating in two different countries where the money exchange rates, railway stamp rate, and postage rates all differed and varied at different times. The postage rate could be handled by selling the stamps appropriate to the country of use. The exchange rate could be handled by accepting only currency of one country. The different railway stamp rates between Iarnrod Eireann and the Ulster Transit Authority required overprinting the stamps in both currencies. Thus, something new to collect. (Figure 4).
For several years, a different overprint was used depending on the destination and routing of the excursion, along with appropriate special cancels.
Another group re-opened the Downpatrick & Ardglass Railway, Downpatrick, Co. Down, in 1990. Their operation also offered railway letter stamps, although not to the extent of the RPSI. (Figure 5). Both groups remain active, and their railtours are very popular.
Great Britain & Ireland Railway Letter Stamps 1957-1998, A Handbook and Catalog, Oakley, Neill, Railway Philatelic Group, 1999.
Irish Stamp News, Vol. 1, Issues 7,11,16,21,26,31; Vol II, issues 2, 5.