To avoid problems with smoke and steam overwhelming staff and passengers on the covered sections of the Metropolitan Railway, Fowler proposed a fireless locomotive. The locomotive was built by Robert Stephenson and Company and was a broad gauge 2-4-0 tender engine. The boiler had a normal firebox connected to a large combustion chamber containing fire bricks which were to act as a heat reservoir. The combustion chamber was linked to the smokebox through a set of very short firetubes. Exhaust steam was re-condensed instead of escaping and fed back to the boiler. The locomotive was intended to operate conventionally in the open, but in tunnels dampers would be closed and steam would be generated using the stored heat from the fire bricks.