If you take a close look at the picture of N Class No.1850 pictured here in Southern livery, you'll notice something odd about the valve gear.. its the invention of Mr J T Marshall who won over the Southern during January 1931 with the idea of trying it out. Although the Southern were keen, they were out of luck as Mr Marshall died. (I guess Marshall was the unluckier its true!). It was 1933 before the drawings for the valve gear were complete and No. 1850 selected to try the system out.
She emerged from Eastleigh works on Feb. 3rd 1934, and underwent trials shortly afterwards. She ran freight turns between Basingstoke and Southampton with N Class No.1413 used as a comparison.
The trials confirmed that the Marshall fitted 1850 had the slight edge over the normal N Class. This didn't seem to out weigh the complexity of the system. The gear tended to "knock" at speeds above 50mph but it's not known if this was the cause of the failure suffered by 1850 en-route for Waterloo, when the right hand gear disintegrated. With the designer unable to promote his cause, the fate of the gear was sealed. 1850 was converted back to normal valve gear arrangements at Brighton.
Information source for this is Kevin Robertson's "Leader and Southern experimental steam" Sutton 1990 ISBN 0 86299 743 7, which is a book I can recommend highly