The locomotives had outside cylinders with inside valves and inside Stephenson valve gear. There were two variants:
Q1/1 (69925-69928) with 1,500-imperial-gallon (6,800 L; 1,800 US gal) tanks
Q1/2 (69929-69937) with 2,000-imperial-gallon (9,100 L; 2,400 US gal) tanks
The extra 500 imperial gallons (2,300 L; 600 US gal) of water in the Q1/2 was carried in the rear tank, under the coal bunker, resulting in a reduced coal capacity.
Withdrawal
All the Class Q1 locomotives were withdrawn and scrapped from 1954 to 1959.
References
^Ian Allan ABC of British Railways Locomotives, part 4 (1948 ed.). p. 56. ISBN978-0-7110-0401-6.