Railway Sub Office

A Railway Sub Office was an office of the British Royal Mail which received mail directly from a rail source for distribution to surrounding districts, without the mail having first been sent to a regional sorting office and forwarding. After the first RSO was established in 1855, over 2000 such designations were made, the post marks bearing the RSO designation after the post-town's name. The designation ceased after 1905 when road transport became easier and more efficient, but many RSO's continued to use their cancellation stamps for many years after.[1]

The letters are frequently but inaccurately explained as 'Railway Sorting Office'.