1998 25p railway letter stamp from the North Yorkshire Moors Railway (NYMR), marking the rebuilding of BR Class 4MT No. 80135.
U/M / Unmounted Mint / MNH / Mint Never Hinged.
About the North Yorkshire Moors Railway
In 1957, the Talyllyn Railway became the first UK railway to issue its own railway letter stamps. Such labels were popular as souvenirs for the railway's visitors, but also performed a postal function, paying for the carriage of mail via the railway's own letter delivery service. Many other preserved railways followed suit with their own issues, including the North Yorkshire Moors Railway (NYMR) from 1981.
As the Railway Philatelic Group explains, however, subsequent changes to the agreement between the Royal Mail, British Rail and independent railways - most recently in 1998 - has seen the number of railways offering a letter service dwindle, with the final North Yorkshire Moors Railway issue appearing in 2000. As of 2021, only the Talyllyn Railway, Ffestiniog Railway and Nene Valley Railway are understood to still be running a railway letter service.
For details of all NYMR issues, see the excellent Philatelic Compendium website.