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Caledonian class 721 “Dunalastair I to IV”
London, Midland & Scottish class 2P
Great Britain | 1896 | 65 produced
No. 723 Dunalastair I
No. 723 Dunalastair I
Railway and Locomotive Engineering, May 1896

The increasing weight of the express trains in the 1980s and 1990s also presented the Caledonian Railway with the problem that inefficient double-headings had to be used more and more frequently. John F. McIntosh, who was chief engineer from 1895, increased the performance of the 4-4-0 locomotives to the maximum that was possible within the limits of physics and engineering. Key to this was the use of a larger boiler that just fit into the gauge of the Scottish routes and was operated at 160psi. The resulting locomotive was named the Dunalastair class after a well-known Scottish clan. It also formed the basis for Belgian tender locomotives with a 4-4-0 wheel arrangement and tank locomotives with a 4-4-2T wheel arrangement, of which 424 were built.

In 1896 15 examples of the actual Class 721 were built, numbered 721 to 735 and later named Dunalastair I. This was followed in 1897 by the numbers 766 to 780 as Dunalastair II and in 1899/1900 by 887 to 902 as Dunalastair III. The latter two series received four-axle tenders in order to be able to cope better with the longer distances in the Scottish expanses. Records of the Dunalastair III show a 32 mile run with a 250 ton ton at an average speed of 58 mph. Between 1904 and 1910, another 19 units followed as Dunalastair IV. From 1910, a total of 21 engines of classes 139 and 43 were built, which had a superheater ex works and, depending on the author, are either also classified in the Dunalastair class or regarded as a separate class.

No. 769 Dunalastair II
No. 769 Dunalastair II
Locomotive Magazine, June 1898

During development, the boiler pressure was increased first to 175 and then 180 psi. Beginning in 1914, when some Series II, III and IV locomotives were retrofitted with superheaters, the boiler pressure was lowered back to 170 psi and larger cylinders were installed. At the LMS they were numbered between 14311 and 14439. While all Dunalastairs in the original version were retired by 1935, the superheated engines survived longer. Of a total of four engines that were taken over by British Railways, the last Dunalastair IV survived until 1958.

VariantDunalastair IDunalastair IIDunalastair IIIDunalastair IV
ManufacturerSt. Rollox
Axle config4-4-0 (American) 
Gauge4 ft 8 1/2 in (Standard gauge)
Dimensions and Weights
Wheelbase23 ft 1 in23 ft 7 in23 ft 10 in
Fixed wheelbase9 ft9 ft 6 in9 ft 9 in
Service weight105,213 lbs118,329 lbs115,809 lbs121,801 lbs
Adhesive weight70,001 lbs77,616 lbs79,968 lbs80,248 lbs
Total weight206,013 lbs205,859 lbs216,608 lbs240,465 lbs
Axle load35,274 lbs40,124 lbs41,226 lbs41,412 lbs
Water capacity4,203 us gal4,954 us gal4,287 us gal
Fuel capacity8,960 lbs (coal)10,080 lbs (coal)
Power sourcesteam
Estimated power700 hp (522 kW)750 hp (559 kW)820 hp (611 kW)
Optimal speed30 mph27 mph28 mph
Starting effort15,099 lbf17,900 lbf18,411 lbf
Power Plant
Driver diameter78 in
Boiler pressure160 psi175 psi180 psi
Expansion typesimple
Cylinderstwo, 18 1/4 x 26 intwo, 19 x 26 in
Grate area20.6 sq ft23 sq ft21 sq ft
Firebox area118.8 sq ft118.7 sq ft138 sq ft145 sq ft
Tube heating area1,284.2 sq ft1,381.3 sq ft1,462 sq ft1,470 sq ft
Evaporative heating area1,403 sq ft1,500 sq ft1,600 sq ft1,615 sq ft
Total heating area1,403 sq ft1,500 sq ft1,600 sq ft1,615 sq ft
Calculated Values
steam locomotive
John Farquharson McIntosh
last changed: 02/2022

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