A822 was sent to Bricklayers Arms where the N1's performance could be compared with the two cylinder N class, and was found to be no real improvement. The main benefit that it did have was, through having smaller outside cylinders, the ability to work the Hastings line with its restricted width tunnels. Largely due to this additional route availability Maunsell decided in 1930 to build five more, but this time using Walschaerts valve gear on all three cylinders and, in line with these five, modifed A822 to the same specification. Low smoke deflectors were subsequently fitted, as with the N class locomotives. They rode well which made them useful for working summer Saturday relief trains to the Kent coast in the days before electrification. Following this electrification the class moved en bloc to Tonbridge where they worked through to Redhill and onto the Brighton line and other parts of the Central Section.

Bulleid made some minor alterations to them, removing the snifting valves and fitting U1 chimneys to the five 1930-built engines.