Locomotive History of the NBR
The NBR built its own locomotives at Cowlairs, and purchased locomotives from external manufacturers. Many of these externally built locomotives were built by Neilsons who were also based near Cowlairs. At Grouping (1923), the most notable locomotives were the C11 Atlantics and the 'Scotts' (LNER D29 & D30), both built for express passenger traffic.
Apart from the 22 Atlantics, the 4-4-0 was the most usual passenger locomotive for medium and long distance work. At Grouping, a few Wheatley 2-4-0 Class E7s were still in service. Various tanks engines (4-4-2, 0-4-4 & 0-6-2) handled suburban traffic and the ubiquitous 0-6-0, tender and tank, handled freight and shunting duties. Neilson-built Y9 0-4-0ST 'Scottish Pugs' handled dock shunting, whilst a couple of unusual 0-4-0 tender locos (LNER Y10) were used for light freight duties.
It was common practice by the NBR to rebuild locomotives, presumably to extend the life of otherwise obsolete machines.
Thank you to Richard Barron for the above information.