These locomotives were a reinforced version of the standard goods locos that James Manson had had built at Kilmarnock a few years earlier. They had larger cylinders and a larger wheel diameter. Modern features were a steam-powered reverser and steam-powered sanders. The first 20 were built by Neilson, Reid & Co., twelve more in 1907 by North British and a further two in 1910 by G&SWR's own workshops at Kilmarnock.

In 1920 Robert Whitelegg had these locomotives rebuilt with new boilers. Since he wanted to standardize the boiler types, these were the same as those on newly rebuilt passenger locomotives with a 4-4-0 wheel arrangement. They received a new ab, but no superheater.

At a time when the Glasgow & South Western needed a much more powerful express locomotive, Robert Whitelegg had just come from the London, Tilbury & Southend as new Chief Mechanical Engineer. He had been an advocate of heavy tank locomotives in suburban and commuter service and had introduced Britain's first 4-6-4T at the LT&SR with his class 2100. Now he even chose this wheel arrangement for his new express locomotive.

Only six were built by North British in 1922. They were the most powerful locomotives of the G&SWR and served as their flagship, incorporating the latest practices in steam locomotive design. The railway successfully marketed them as the “Baltic Class”, while the crews called them “Big Pugs”, since they were tank locomotives. 

Shortly after their introduction, they became part of the LMS. With increasing traffic on the former G&SWR network, the LMS introduced a larger number of smaller tender locomotives for express service. Since there was now little to do for these large tank locomotives, they were withdrawn in 1935 and 1936 once their boiler was due for renewal.