Joe Mackle immediately established the RMS all over the GNR's operating territory. His strategy included the purchase of many independent bus operators. By the end of 1931 the highly individualistic and ambitious Mackle had built up the RMS to a fleet of 128 buses. In 1933 Mackle moved on to Belfast Corporation Tramways as Rolling Stock Engineer and was later to become General Manager of that undertaking until his retirement in April1963.

In recognition of the fact that he could not do all the work himself Mackle recruited another World War One veteran and holder of the Military Medal - Albert Switzer. Albert was recruited in Dundalk during 1929 as Chief Inspector Road Motor Services. The energetic and peripatetic Albert had an office at Amiens Street Station in Dublin from where he ensured the efficient running of all GNR buses through his Garage Inspectors at principal locations. Albert Switzer greatly relied upon by the company management enjoyed a long and distinguished career with the GNR before his retirement from Coras Iompair Eireann in 1959.

The take over of the small independent bus operators by the RMS between 1929 and 1946 yielded the company about 130 buses together with the operations of 32 companies. These ranged in size from a single bus concern to that of the Irish Free State operations of HMS Catherwood, which brought 24, Leyland buses into the GNR fold. At its zenith in 1935 the RMS ran about 180 vehicles and was rapidly demonstrating to shareholders in the railway, that bus operations were an important contributor to the dividend. By this time the RMS buses operated north of a diagonal line between Dublin and Sligo including the GNR area of Northern Ireland.