Notes

The first two of these engines were introduced to assist two smaller engines working on the 1 mile 35 chain long rack section of Mount Morgan Line. A second pair was later ordered to replace the older smaller machines and then a further two to handle anticipated increases in traffic.

Like there predecessors, the 6D11½ Abt class, they were built with outside frames.

Instructions prescribed that all trains traversing the rack section in either direction must have a rack engine attached in addition to the train engine. Ascending trains were banked in the rear and the rack engine was attached in front of descending trains. At times of heavy traffic congestion two rack engines occasionally worked trains on the incline without the assistance of an adhesion engine

The engines were attached to Mount Morgan and their working was restricted between there and the bottom of the rack section, a total distance of 4 miles. They had the most restricted working area of any QR engines. All were originally assembled at Rockhampton and travelled to there for workshop overhauls.

Maximum permissible speed for the class was 15 MPH.

N° 402 was written off in June 1926 and the remainder were scrapped when the rack section was replaced by a deviation in 1952. The engines were cut up at Rockhampton.

Engines were also fitted with Le Chatelier counter pressure brake. It has been said that boiler pressure was reduced due to excessive slipping of the adhesion engines

Abbreviations

Dubs – Dubs & Co. Glasgow

NBR – The North British Locomotive Co. Ltd. Glasgow