From 1846, John Gray had twelve 2-2-2 express locomotives built for the LB&SCR, which are considered the forerunners of the Jenny Lind type. Although they had outer and inner frames, the driving axle with the six-foot wheels was only mounted on the inner frame. They already had a firebox with a round, raised ceiling, on which sat a second steam dome. They were said to be able to easily reach a speed of 40 mph with the train weights that were common at the time. Under Craven, all were converted to a different wheel arrangement in order to collect experiences with the properties of the different types and the respective suitability for different areas of application.

During the first conversion in 1853, four examples were converted into freight locomotives with a 2-4-0 wheel arrangement. The diameter of the coupled wheels was now four feet and nine inches. Later, three of them were converted into tank locomotives with 0-6-0 wheel arrangement by exchanging the leading axle for another coupled axle with the same wheel diameter. 

Between 1853 and 1855, six other engines were converted into passenger locomotives, which, like the freight locomotives, had 2-4-0 wheel arrangement. However, these had a coupling wheel diameter of five and a half feet. The fact that none of these were rebuilt again and remained in use until the 1870s suggests that they were considered successful in this form.

The other two locomotives were converted to the Crampton design, which was a special design with a crankshaft. At the front end, the two carrying axles sat a short distance apart in the frame and behind the firebox was the driving axle, the wheels of which were still six feet in diameter but clad in a wooden wheel disc with no visible spokes. The internal cylinders worked on a crankshaft and drove external coupling rods. Both locomotives only ran in this form for about two years and were converted into 2-4-0 freight locomotives in 1855. One of these two was converted into a tank locomotive with the same wheel arrangement in 1858 and remained in service until 1875.