However, that was all set to change in 1908. The latest class of Atlantic tank, the I3, had first ventured out of the workshops in the form of N°21 in September of 1907 and was, basically, a tank engine version of Billinton's successful B4 4-4-0 with 6' 9" wheels, 19" by 26" cylinders and a 180 lb sq ft saturated boiler. This was a great improvement on those that went before, but was still lacking that little bit "extra" that would make it a really useful engine. The next I3 to be built was N°22 in March 1908 and for this Marsh was persuaded by his Chief Draughtsman to fit a superheated boiler with an extended smokebox on a saddle, 6' 7½" wheels and 21" by 26" cylinders. Although not the first superheated locomotive in Britain (both the L&YR and the GWR had experimented with superheating) it was the first superheated express engine and without doubt the one that showed superheating was the way forward. Trials with N°22 showed it to be economical on both coal consumption and maintenance and a powerful performer on the heaviest of trains. Five more superheated I3s were built during 1908, then during 1909 and 1910 six more saturated versions were built so that a truly faircomparison could be made between the saturated and superheated boilers. When compared with saturated I3s, H1s and B4s the superheated I3s consumed 30lb of coal per mile as against the 36, 40 and 42lb of the others. Furthermore, they were capable of operating the heaviest of LB&SCR expresses over all routes without problem and could work the 84 miles from Clapham Junction to Fratton in 1 hr 40 mins non-stop, despite having just 2,110 gallons water capacity. In one trial N°23 worked the "Sunny South Special" through from Willesden Junction to Rugby much to the astonishment of the L&NWR that usually provided two engines for this duty, a fairly lightly loaded 250 tons!

The main thing to make the other railways sit up and take notice was the fuel economy, though they were not lacking in awareness of the other benefits. A superheated I3 could leave Brighton with a piled high bunker containing some 3¼ tons of coal and take the above-mentioned train to Rugby and back without need for water en route or recoaling at Rugby, a performance of about 27lb per mile, assuming no coal remaining when returned to Brighton. The remainder of the class was built with superheated boilers (the saturated ones were rebuilt as superheated by Maunsell) and the I3s ruled supreme until electrification pushed them to the secondary routes of the Crowborough, East Grinstead and Tunbridge Wells lines.

Numbers 21 to 81 were built under Marsh, the remainder under LB Billinton. This last 10 were slightly different in that they were provided with the vacuum brake and were fitted with different feed pumps, a higher cab roof and no superheater dampers. This dual brake facility made the Billinton engines very useful for working ambulance trains during the First World War.

The hero of the day was BK Field, Chief Draughtsman, whose persistence was rewarded when he finally persuaded Marsh to try superheating, though few today will have heard of him, and even fewer recognise his contribution to the development of the British steam locomotive.

The class had a very successful career and when electrification of the Brighton lines started to displace them they found new jobs elewhere. Initially cut down slightly by reducing the height of the boiler mountings and rounding-off of the cab roofs to work on the Eastern section, they subsequently moved further afield with, from 1938, four being allocated to Salisbury shed to work trains from there to Portsmouth and, in 1941, two going to the GWR at Worcester where they easily handled the traffic normally worked by that company's Prairie tanks. By 1943, however, all of the class had returned to the Central section.

Unfortunately none of these mould-breaking locomotives survived the cutter's torch with the last, N°32091, being withdrawn in April 1952 and cut up at Ashford the following January, still making a name for the class as N°32091 was the first ex-LB&SCR express passenger locomotive to be cut up there.