As the railways established themselves as a reliable method to transport goods one problem came to the forefront – fire. Steam locos fundamentally need a heat source created from some form of fire to turn water into steam. However, this fire, alongside sparks emitted from the chimney, caused great concern in environments where munitions, fuel or other flammable materials were being moved or stored.
The solution is a locomotive that won’t spark, or have a naked flame on board. Thankfully, some bright spark had the clever idea of harnessing steam power in a large rechargeable steam reservoir, which would sit in the place of the boiler of a conventional steam locomotive. This reservoir would be filled with a mix of boiling water and steam at high pressure. As the locomotive was used the steam would be used and thus the pressure in the reservoir would drop. This in turn allowed the water to start to boil replacing the steam that had been used. With this method a locomotive could work for about 4 hours prior to recharging.
The result is a design that has a distinct lack of key features which people associate with more traditional steam locomotives, such as a smokebox and chimney, a firebox, forward-mounted cylinders and coal bunkers or tenders. Its lack of a firebox requires its cylinders to be rear-mounted both to improve efficiency and redistribute the weight of the loco, and its other missing features are simply because they are surplus to requirement.
