Royal Mail, the photograph shows an old Royal Mail train which was in use up until the 1970’s i believe. The unit itself is in great condition and can be viewed at Bressingham Steam Museum, UK.
A Travelling Post Office (TPO) was a type of mail train in Great Britain where the post was sorted en route. The last Travelling Post Office services were ended on 9 January 2004, with the carriages used now sold for scrap or to preservation societies.
Following an agreement in 1830, made between the General Post Office and the Liverpool and Manchester Railway (L&MR), mail had been carried by train in Great Britain, between Liverpool and Manchester, via the L&MR. The passing of the Railways (Conveyance of Mails) Act 1838 required railway companies to carry mail, by ordinary or special trains, as required by the Postmaster General; however this act did not set the charges for such services.
These special trains eventually became Travelling Post Offices (TPOs). TPOs were employed in many British Commonwealth countries; and the Army Post Office had its own TPOs.
TPOs were equipped with letter boxes so that mail could be posted whilst the train stood at a station. The post-marks from TPOs are valued by philatelists – Wikipedia