The Franco-Crosti system recycled exhaust steam from the cylinders through a second 'boiler' fitted between the frames and below the conventional boiler. This was actually a water pre-heater barrel, where waste heat from the steam was utlised to heat water before it was pumped into the boiler proper. The exhaust steam then exited via the flat 'chimney' outlets alongside the boiler, instead of via the conventional chimney at the front of the engine.
While pre-heating water before it is fed to a boiler obviously reduced the fuel needed to generate steam and using waste heat from steam which was about to be discharged from the engine is equally obviously better than just throwing it away there were disadvantages. Compared to a conventional 9F, which possesed a new and quite efficient boiler of modern design, the additional cost and complexity in maintenance of the pre-heater drum quite easily negated the quite small fuel savings which were achieved under test.
The Franco-Crosti boilered 9Fs were converted to conventional operation, but continued to carry their slightly small boilers and the evidence of the pre-heater 'boiler' below it until withdrawn.