Many of the superheated locomotives were also later returned to saturated steam as their original boilers wore out.

The first batch of ten locomotives were built at Darlington Works between June and November 1886.

Six out of these seven were also fitted with superheating between 1919 and 1930, raising the weight to 59 LT 10 cwt (60.5 t), although in two of these cases they were returned to saturated steam in 1941 and 1944.

9) was constructed at Gateshead in January 1888, but fitted with the Von Borries compounding system.

As with the other members of the B1 class many of the locomotives were with superheating between 1916 and 1927, although in several cases they were returned to saturated steam between 1941 and 1950.