Glasgow & South Western class 540

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 Since there was now little to do for these large tank locomotives, they were withdrawn in 1935 and 1936 once their boiler was due for renewal.