The PT class was first catalogued in the second edition of the BESA report on standard locomotive classes for the railways of British India of 1907.[6]Robert Stephenson and Company built seven PT class locomotives for the South Indian Railway (SIR) in 1936, bearing works numbers 4114 to 4120.[2] This batch differed significantly from the earlier F and FS classes (which correlate to the original BESA design with internal valve gear) by being fitted with outside cylinders.[1] In service, the locomotives were used in heavy suburban traffic, and were numbered 7 to 13.[2] Number 11 has been preserved and is on display in the National Rail Museum of India in New Delhi.[7][8]
Drawing of the original design from the BESA's second report from 1907 (note the lack of external cylinders).
Three design variants of the PT class, each with different boilers, were listed in the BESA report. A larger standard boiler with a diameter of 5 ft 1 1⁄4 in (1556 mm) was developed for the PT, which was also listed as a variant in the third BESA report for the SP and SG class locomotives.[9] Alternatively, the existing boilers from the SP and SG classes could also be used, provided that the weight of the locomotive was altered in accordance with the requirements of the report.[6] The PTS variant was fitted with a Schmidt superheater - the S stands for superheated.[5] In contrast to the design recommendations of the BESA report, the SIR locomotives were fitted with external Lentz rotary valve gear.[1]
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