Historically, mail was carried by horse drawn coaches between main centres of habitation. When railways appeared in the 1830's mail was progressively transferred to the trains and, basically, they followed the same routes as the mail coaches, ie town centre to town centre. Originally, mail was just carried in bags but it was quickly realised that mail could be sorted on the trains and, thus, Travelling Post Offices (TPOs) were established. In the late 1850's it was decided that, to speed up the delivery of mail further, it would make sense to send mail directly to towns and villages which the TPOs passed on their journeys. These offices were then designated as Railway Sub Offices (RSOs) and, as such, could receive and/or despatch mail direct to or from a TPO, without it needing to go through the Head Office. This arrangement was restricted to England, Wales, Scotland and the whole of Ireland. There are some very interesting locations that became RSOs. These include islands that had no TPOs such as Skye and the Orkneys. These, fairly easily, can be explained as the bulk of their mail was received from the Highland TPO where sealed bags would be despatched direct from the train to the islands. However, there are other islands such as Mull, Islay and Arran that had RSO offices on them but there were no connections with TPOs. It would appear that these offices received their mail via MacBrayne's Steamers. It is certainly known that at least two of these had Post Offices on them but more research in this area is needed. Postmarks incorporating the letters RSO could have been issued to all offices that were designated as RSOs but it would appear that this did not happen. Virtually all types of postmarks have been recorded incorporating RSO including duplexes, double circles, single circles, skeletons and dumb parcel cancels (but not squared circles). However, it is known that duplexes were issued to certain offices when they became RSOs and their hand stamps did not include the letters RSO and this probably applies to other types.

There is, therefore, a whole study area of postmarks used at offices when they were designated as RSOs but not including these letters in the postmarks. Also, postmarks are recorded for sub office of RSOs, including temporary rubber date stamps which include the designation, eg Linkinhorne was a sub office of Callington RSO and a single ring LINKINHORNE/CALLINGTON.RSO was used. Also, postmarks are recorded for sub offices of RSOs that include the designation, eg Elsdon was a sub office of Otterburn RSO and a single ring ELSDON/OTTERBURN.RSONORTHD used. In addition to postmarks, offices that were designated as RSOs have used Parcel Post Labels that incorporated RSO in their central description. These are quite scarce and very collectable. When the Post Office announced that an office was to become a RSO its address was changed to incorporate RSO into it and the Head Office was dropped, eg when Lynmouth became an RSO on 1 March 1903, its address changed from Lynmouth, Barnstable to Lynmouth RSO, North Devon. This was, effectively, an instruction to the Sorting Office Staff that a separate bag was to be made up for this office and items were not to be included with mail for its previous head office. Thus, RSO became part of the address of each such office and this appears on all sorts of documents, such as letterheads, invoices and similar which, therefore, are collectable items.

RSOs ceased to exist, officially, on 1 August 1905. By this date there was a properly surfaced road system in place with mechanised road-going vehicles and, hence, it was not as necessary to use all the previous methods to speed the mail. However, the basic system continued for many years and postmarks including RSO were still in use as late as 1944. As cancellers were repaired after the office ceased to be an RSO, sometimes the 'R' of 'RSO' was deleted and either a space or a dash put in its place. There were at least 313 RSO offices in Wales, 435 in Scotland and 794 in England but work is still ongoing to establish the number in Ireland.