Based on his class A three-cylinder compound, Francis Webb designed the four-cylinder compound class B for the LNWR. Between 1901 and 1904, a total of 170 were built. They earned the nickname “Swami” after magicians who used to disappear in a cloud of smoke. Like their predecessors, they had clearly visible inclined outside cylinders, but here these were smaller. Due to the four cylinders, they were fairly top-heavy with 31 tons on the front two axles and only 22 tons 10 cwt on the rear two axles.

So the numerous rebuilds of class B started when George Whale rebuilt 26 with a leading axle and designated them class E. Ten class F locomotives rebuilt between 1906 and 1908 also had a leading axle, but also a bigger boiler. 32 two-cylinder simples rebuilt between 1912 and 1927 were designated class G. Here the two bigger inside low-pressure cylinders were used as new high-pressure cylinders.

Locomotive Magazine, March 1902

Another more comprehensive rebuild took place between 1917 and 1927 when Bowen Cooke rebuilt 91 class B locomotives to the superheated two-cylinder class G1. Additional locomotives of classes E, F and G and even the rebuilds of the class A were also rebuilt to class G1. Some even lost their leading axle again when the outside cylinders were removed. So the LMS had only taken over 53 unrebuilt locomotives of class B in 1923, but rebuilt more of them. In 1927, only ten unrebuilt locomotives were left over which were withdrawn within a year.