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As is usual in engineering, there were a number of trade-offs to be made in steam locomotive design. There is a limit to the rate that steam can be delivered to the pistons, therefore higher speed was obtained with larger driving wheels. These however limited the size of the boiler, since it needed to fit between them, particularly with the preoccupation of the time with a lower centre of gravity. The tendency had been to lengthen the boilers with supporting wheels front and rear. Thus passenger engines, such as the so-called Long Boiler locomotives were usually of a 4-2-0 wheel arrangement. However too long a boiler also created instability.
Some locomotives improved adhesion for heavier loads by coupling pairs of driving wheels, however there was a tendency for the coupling rods to break especially at speed. Thus four-coupled locomotives were used for freight trains.
David Joy, as Chief Draughtsman, is usually credited with its design. Other claims were for James Fenton or Edward Brown Wilson. Fenton was the person who started the company, and Joy's manager, so clearly it was a development of his designs, while Wilson was the owner of the company at the time, and could be seen as more of an entrepreneur and salesman. Joy had spent his formative years studying all the locomotives he came across, sketching them, making notes, and interrogating their owners and crews and, if he could, getting rides on them.
He settled on a medium sized boiler, 10 ft. 6 in. in diameter, and concentrated on its steaming abilities. The engine had 15 inch by 20 inch inside cylinders and 6 foot diameter driving wheels. The design also owed much to John Gray's locomotives for the Brighton Railway. The so-called 'mixed' frame had an inside frame for the cylinders and driving wheels, with inside bearings, and an outside frame for the four foot diameter leading and trailing wheels, using outside bearings. The inside frame stopped at the firebox, so that the latter was as wide as the wheels would allow. By this means he minimised the overhang at each end.
After some strengthening of various members, the engine was three tons heavier than expected. However, it steamed freely and was economical on fuel. It was to this that its success was attributed, along with the increase in boiler pressure that had become possible over the years. However credit must be given to Joy's suspension arrangements that made it extremely smooth-running and stable.
The name Jenny Lind was given to the first one delivered to the London and Brighton Railway, but it was such a success that it became the name for the whole class of which seventy were built for various railways, including twenty-four for the Midland Railway. It could be said to be the first to be mass-produced to a consistent pattern. Indeed the manufacturers charged a hefty premium for variations, although in response to pressure, they later built a number of 'large jennies'.