problems, whilst retaining the good features, thereby creating excellent locomotives. One minor drawback was that the 'modifieds' put greater loads on the track as a result of hammerblow, caused by the balance weights for the outside Walschaerts valve gear, whereas the original valve gear design was largely self-balanced. On 26 June 1967, 35003 Royal Mail recorded the highest speed ever for the class. Hauling a train comprising three carriages and two parcels vans (164 tons tare, 180 tons gross) between and Waterloo, the mile between milepost 38 and milepost 37 (located between and ) was covered in 34 seconds, a speed of 105.88 mph. This was also the last authenticated speed in excess of 100 mph achieved by a steam locomotive in the United Kingdom, until the same mark was attained in 2017 by Tornado. Accidents and incidents On 17 December 1942, No. 21C6 Peninsular & Oriental S. N. Co. sustained a broken chain near . The sump was fractured leading to an oil fire. On 7 October 1943, No. 21C1 Channel Packet sustained a broken chain at . The sump was fractured leading to an oil fire. On 29 January 1945, No. 21C12 United States Lines sustained a broken chain. On 24 April 1953, the crank axle on the central driving wheel of No. 35020 "Bibby Line" fractured whilst approaching Crewkerne station at speed. No-one was injured, but the incident resulted in the withdrawal of all Merchant Navy class locomotives from service whilst the cause was ascertained. An examination of other class members showed that the fracture, caused by metal fatigue, was a common fault. To cover the motive power shortage caused by the mass withdrawal of thirty locomotives, classes from other British Railways regions were drafted in to deputise. The incident resulted in a redesign and replacement of the crank axle. Withdrawal Their principal work was on the South West Main Line to Southampton and Bournemouth until 1967. However, the main reason why the class began to be withdrawn in 1964 was the transfer of the main line between Salisbury and Exeter to the Western Region and the introduction of "Warship" class diesel-hydraulic locomotives on these services. The rebuilt locomotives were therefore withdrawn relatively soon after their rebuilding, whilst still in excellent condition. The first two to be withdrawn were the second prototype 35002 Union Castle and 35015 Rotterdam Lloyd in February 1964. Nearly half of the class