London & North Western Webb Experiment class

In 1881, Francis Webb started to design an express locomotive without coupling rods for better running characteristics at speed. He created a three-cylinder compound system that used two very small high pressure cylinders on the outside and a large low pressure cylinder on the inside. The two smaller ones were located behind the leading axle and drove the second driving axle, while the larger one was mounted below the smokebox and drove the first driving axle. Both had different types of valve gear, Joy on the outside and slip-eccentric on the inside.

The prototype was No. 66 “Experiment”, built at Crewe in January 1882. It hauled the fast mail trains to Ireland and Scotland and covered more than 300 miles per day. Webb claimed that it used 30 percent less coal than a standard 2-4-0 locomotive. As it was found out that the high pressure cylinders were too small, their diameter was increased from 11.5 to 13 inches, while the high pressure cylinder still had a diameter of 26 inches. 29 more were built between February 1883 and July 1884. Although they were not very successful, the larger Dreadnought class was based on them. Both classes were withdrawn between 1903 and 1905.