In 1955 BR experimentally built 10 9F locomotives which incorporated Franco Crosti boilers, resulting in an engine that was both striking and workmanlike in appearance. The Crosti's were about 4 tons heavier than standard 9F's and initially allocated to 15A Wellingborough for use mainly on heavy midland's coal traffic. At time they ventured far and wide.
With a tractive effort of 39670 lbs and 5' driving wheels all the 9F's were powerful and capable engines. As to the Crosti's, the hoped for improvement in steam production and hence performance didn't quite meet expectations and no more were produced. Adapted in various ways, the ten produced remained an interesting sub section of the 9F class. Of the latter 92220 Evening Star was the last steam engine to be built by British Railways.
The large class 9F 2-10-0 locomotives were born out of British Railways' requirement for a heavy freight locomotive and was part of their standardisation plan. By 1954 the first twenty Class 9F locomotives had been introduced, however with coal economies in mind it was decided as an experiment that the next ten of the Class should be built using Franco-Crosti boilers, which had demonstrated significant reductions in coal consumption on modified loaocmotives in Italy.
Named after two Italian engineers who had worked for the Italian State railway in the 1930s, the Franco-Crosti boiler was a combination of a conventional fire tube boiler used on the majority of steam locomotives with a large capacity feed water preheater. Instead of being exhausted directly through the conventional chimney (which was used for lighting-up and initial steam pressure raising) exhaust steam and combustion gases were redirected through the preheater barrel to exit though the exhaust system on the side of the locomotive. Residual heat from the exhaust steam, which would otherwise have been wasted, was utilised to heat the water in the preheater drum, raising its' temperature before the water entered the conventional boiler. Less heat and therefore fuel was then required to keep the locomotive boiler at working temperature and pressure.
While the Franco-Crosti boiler and indeed other feedwater heater systems which recover heat which would otherwise be lost obviously provide improved efficiency and this was significant with older boiler designs the BR 9F carried a modern design boiler incorporating many other proven improvements. To accomodate the preheater barrel the Crosti 9F main boilers had to be reduced in size, the combination of the already remarkably efficient boiler of the standard 9F locomotive and smaller-sized boilers of the Crosti engines significantly impacted the measured improvements in British Railways service. By the end of the 1950s diesel traction was clearly identified as the way ahead for British Railways and further experimentation with steam powered locomotives ceased, leaving the Crosti boilered engines to work out their time as conventional locos, with the preheater barrels blanked off.