Nationalisation had been kind to the Bulleid Pacifics and, indeed, steam in general, because it delayed the electrification schemes proposed by the erstwhile SR. In fact, in its early years, BR’s attention turned towards developing a completely new range of relatively maintenance-free engines, the ‘’Standard Classes’’. The Kent Coast lines, which the SR had hoped to electrify by 1955, were not authorised for such works until February 1956. During the 1950s, however, a case had been made against running the Bulleid Pacifics in their original ‘’air-smoothed’’ formed and rebuilding the engines with a more conventional appearance, in line with the BR Standards then being produced. The innovative Bulleid features made his Pacifics expensive engines to operate and, at times, gave them poor availability, the latter contrary to what he intended. In 1954, Ronald G. Jarvis, the Chief Technical Assistant of the Locomotive Department, Brighton, was put in charge of a scheme to improve these engines. Jarvis was formerly a Derby man, and the prevailing BR culture was one of keeping costs low and running engines which were both relatively cheap and easy to maintain.
The decision was made to rebuild every Bulleid Pacific – 30 MN engines and 110 Light Pacifics. This was at a time when steam was intended to remain as BR motive power up until 1990, after which electric traction would take over. Cosmetic alterations to tenders had earlier begun in 1952: the high side flanges, which extended up to the same level as the cab roof, were cut down, to enable a water column pipe to be swung over the tender side freely. Then, in 1954, the boiler pressure of both MN engines and ‘’Lightweights’’ was de-rated from 280 PSI to 250 PSI. Up until this time, the Bulleid Pacifics and the ex-GWR ‘’County’’ engines shared the title of having the highest boiler pressure rating of any steam locomotive class in the country. Reducing the pressure allowed a more common grade of steel to be used in the firebox, which in turn made the engines cheaper to maintain. As earlier mentioned, the boilers of these engines were of welded construction, which was particularly important for the Light Pacifics, because it kept weight down. Steel tubes, called ‘’thermic siphons’’, were incorporated into the firebox: water was pumped through these, which provided a greater overall heating surface area. The raison d’être of this design feature was to conserve fuel, whilst at the same time raising steam production – as figures reveal, these engines were somewhat heavy on coal consumption.
The rebuild involved eliminating the chain-driven valve gear and oil bath, and incorporating the more common Walschaerts’ system. The steam reverser was removed in favour of a screw reverser, and the inside cylinder replaced by a completely new design. In brief, the steam reverser was linked to the valve gear and controlled the admission of steam to the cylinders through regulating the timing of the valves. Whether the locomotive moved backwards or forwards was determined by steam being sent into the cylinders above or below the pistons. Visually, the greatest alteration was the removal of the ‘’air-smoothed’’ casing and fitting of conventional boiler cladding. Again, emphasis was made on bringing these engines in line with the BR Standards then being brought into service. Removal of the casing provided improved access to boiler fittings, and room was now available for step-boards above the wheels. BR-style smoke deflectors were also fitted. No. 35018 ‘’British India Line’’ was the first engine selected for rebuilding, and was admitted to Eastleigh works on 16th November 1955. The work was completed over a three-month period, and No. 35018 was outshopped in its new form and put on display at Waterloo’s platform 15, on 14th February 1956. No. 35028 was the final of the MN Class to be rebuilt, entering Eastleigh Works in October 1959.
Decline and Western Region Intervention
As of 1959, three MN engines were allocated to the South Eastern Division, all based at Stewarts Lane: the doyen of the class, No. 35001 ‘’Channel Packet’’; No. 35015 ‘’Rotterdam Lloyd’’; and No. 35028 ‘’Clan Line’’. In addition, ten were based at Nine Elms, seven at Bournemouth, seven at Exmouth Junction, and finally, three at Salisbury. The start of full-accelerated electric timetables on Kent Coast Lines (first the main ‘’Chatham’’ line on 15th June 1959, followed by the ex-SER trunk route via Tonbridge and Ashford on 18th June 1962), meant that steam became virtually extinct on the South Eastern Division. The remaining Bulleid Pacifics were transferred to the South Western Division, where steam traction had yet to be fully ousted. A large blow to the SR Pacific fleet came on 15th September 1962, when all Southern Region lines west of Salisbury were brought under the control of Paddington. Thus, all those engines based at Exmouth Junction, including the seven MNs, were absorbed into the Western Region fleet. Of those MN locomotives, just one, No. 35009 ‘’Shaw Savill’’, was withdrawn by the WR during the elimination of steam traction in the Exeter area, in September 1964. Happily, the other six members made their way back to the Southern Region, all but one (No. 35011 ‘’General Steam Navigation’’) remaining in service until summer 1967.
In September 1964, the Minster of Transport authorised the electrification of the South Western Division main line from Brookwood to Branksome, Dorset (just west of Bournemouth), on the familiar third rail system. This would completely eliminate steam from the SR; beyond Bournemouth, diesel-electric traction would be used. Earlier, in February of the same year, the first MN engines had been withdrawn from service: Nos. 35002 ‘’Union Castle’’ and 35015 ‘’Rotterdam Lloyd’’. By the start of 1965, Nos. 35001, 35006, 35018, and 35025, had also gone. Full electric working to Bournemouth was planned to commence on 10th June 1967, but was put back by exactly a month following delays in the delivery of electric rolling stock. 9th July 1967 was the final day of steam working from Waterloo, by which time seven MN engines were still in service: Nos. 35003, 35007, 35008, 35013, 35023, 35028, and 35030.