On the 'jockey valve' by County1006 on DeviantArt (original) (raw)

6960 'Raveningham Hall' coasts downhill at Woolston Moor on the 12.35 from Bishops Lydeard, Somerset, on June 24, 2015. 6960 was actually a true Great Western 'Modified Hall' being built in 1944, and the GWR continued to build them up to about 6980 whereupon British Railways took over the build to 6999 and then 7900 to 7929.

The 'Halls', in common with GWR locomotives in general, had what was known as a 'jockey valve' operated by the regulator. When the locomotive was coasting, as here, the driver would not close the regulator fully, but keep it 'cracked' (the 'coast' position). As the displacement lubricator could not be effective when coasting, the jockey valve would operate with the regulator cracked to feed some oil though to the cylinders with the steam. I don’t think the other railways had this feature.