Just five locomotives were built which were almost identical to the I2 class but with superheating, the cylinders' diameter increased from 17½ ins to 20 ins and the boiler pressure reduced to 160 lb. Sadly it was soon proved that the superheating alone was not the key to the success with the I3 tanks and the I4 engines were a big disappointment! In fact, in his book on the LB&SCR, Hamilton Ellis goes so far as to describe them as being 'shocking'!

The Southern Railway inherited these beasts and didn't know what best to do with them. There was a proposal to rebuild them along the lines of the I1x rebuilds of the I1 class, but in the end opted to scrap them. All bar one had gone by 1937, with the survivor, N°34, hanging on until 1940. It is surprising that the creator of the H1 and H2 classes (albeit basically a copy of the Great Northern Atlantics) and of the I3 class made such a poor job of his other designs.

It is somewhat ironic to note that whilst Marsh was still building dismal 4-4-2Ts at Brighton, based on his experiences with the GNR, the GNR was busy introducing successful 0-6-2T locomotives that were, in turn, based very much on the 1902 E5 class of Marsh's predecessor, Billinton!