The UTA was formed by the 1948 Transport Act (NI), which merged the Northern Ireland Road Transport Board (NIRTB) and the Belfast and County Down Railway (BCDR). Added to this in 1949 was the Northern Counties Committee (NCC), owned by the Railway Executive since its previous owners, the London, Midland and Scottish Railway (LMS), had been nationalised into British Railways in 1948.
Quite substantial closures took place, including all but one line of the Belfast and County Down Railway. In 1958 the Great Northern Railway Board (GNR(I)), one of two remaining railways that crossed the border between the Republic of Ireland and Northern Ireland, was split between Córas Iompair Éireann (CIE, the Republic of Ireland's nationalised railway system) and the UTA. The UTA took over the GNR system in Northern Ireland, but by 1965 they had closed down more than half of it.
Transport Acts in 1967 saw the organisation split into road and rail operations, the bus operations being taken over by a new company called Ulsterbus in the same year. The rail operations temporarily became Ulster Transport Railways (UTR) before being taken over by Northern Ireland Railways (NIR) in 1968.