- British Rail Class 74
name = British Rail Class 74
powertype = Electro-diesel
caption = 74 003 in BR blue with full yellow ends
British Railways’ Doncaster Works; rebuilt BR Crewe Works
builddate = 1958–1960; Rebuilt 1967–1968
gauge = RailGauge|ussg|lk=on|abbr=on
totalproduction = 10 (rebuilt)
wheeldiameter = convert|4|ft|0|in|m|3|abbr=on
electricsystem = 650–750 V
collectionmethod = Third rail
primemover = Paxman 6YJXL
English Electric532A, 4 off
tractiveeffort = "Electric:" convert|47500|lbf|kN|1|abbr=on|lk=on
whytetype = Bo-Bo
uicclass = Bo'Bo'
trainbrakes = Vacuum, Air, Electro-Pneumatic
weight = convert|77|LT|lk=on
poweroutput = "Electric (continuous):" convert|2300|hp|0|abbr=on|lk=on
"Electric (one-hour):" convert|2552|hp|0|abbr=on
Electric Train Heating
British Railways’ Southern Region
roadnumber = E6101–E6110
disposition = All scrapped
British Railclass 74 was an electro-diesel locomotivethat operated on the Southern Region of British Railways, rebuilt from redundant class 71 locomotives in the late 1960s. An electro-diesel locomotive is one that can operate either from an electrical supply, such as overhead catenary or an energized 3rd rail, or from an on board diesel engine.
Twenty-four examples of
British Rail Class 71(pre-TOPS type HA) were built in 1958 at the British Railworks in Doncaster, and in 1964 ten were deemed surplus to requirements, withdrawn from service and placed in storage. The Southern Region was highly impressed with the "little-ED" JA & JB/class 73 locomotives and were keen to see a higher power locomotive with the same flexibility. In 1965, discussions were opened with English Electric (builders of the production JB/class 73) to produce a high powered electro-diesel. Originally a centre-cab locomotive was envisaged similar in concept to the Clayton class 17 but details on this are scant. This design was not pursued - possibly due to the historical frugality of the SR and the availability of the ten moth-balled members of class 71. These were moved in groups to the British Rail works at Crewe, where they were rebuilt into type HB/class 74 electro-diesels. Buck-eye couplers and high-level control & brake jumpers were fitted to facilitate working with other EP stock - especially TC units in push-pull mode. Also, two-tone "raspberry" air horns were mounted on the cab roof, replacing the original air whistle of class 71.
They were intended especially for use on the
Southamptonand Weymouthboat trains, whose routes included sections of non-electrified track (with some tramway along the public thoroughfare). The elimination of the locomotive change (at either Southampton or Bournemouth) was envisaged, and their dual power capability would greatly accelerate timings and reduce operational requirements.
Originally, plans had been to number them E7001-E7010, but once rebuilt they were numbered E6101-E6110 instead. Later they were renumbered 74001-74010 under the TOPS system.
In common with the first SR DC electric locomotives (class 70), class 74 utilised a 'booster' set (a motor-generator and flywheel combination) to overcome the problem of gapping. Booster set 836/2D had been designed by English Electric for use in class 71 and this was retained in the rebuild, although weight considerations meant that the flywheel assembly, separate auxiliary generator and traction motor blowers were removed. The compact size of the booster permitted a small diesel engine and generator inside the body shell (only a single booster was used in classes 71 & 74 unlike the two of class 70). Thus, these locomotives could operate either from a third rail supply at 650 V DC (Eastern & Central sections), 750 V DC (Western section) or from their Paxman 6YJXL 'Ventura' diesel engine, pressed to just 650 hp - well within the engine's stress curve and thus greatly extending service intervals. This choice of engine (despite its problems as employed in class 74) was a good move. 57 of these engines had been supplied for use in class 14 and a further 20 had been supplied to Scottish workshops of BR for the re-engining program of class 21 (such locomotives being designated class 29). Thus a proven track record and a plentiful supply of spares was assured. The class 71 pantograph for overhead current collection was dispensed with.
Electronic traction control
Obtaining a DC supply controlled by rectifiers requires an AC power source. Consequently, the existing auxiliary generator was converted to a three-phase alternator with automatic voltage regulator. This arrangement gives improved control of traction current over conventional rheostatic systems and gives fine control when starting a train. Normally the driver has to maintain tractive effort well below the rail adhesion limit to give himself time to respond to wheel slip. The constant current systems of class 74 were designed to allow the driver to apply power very near the limit of adhesion and the booster output is regulated in such a way that the maximum current of any traction motor does not exceed the selected value. Thus, even when one motor (axle, wheel set) starts slipping, the voltage across the whole group of motors cannot rise because current in the non-slipping motors is fixed. Simply speaking the motors all work as a differentially balanced team so that as one motor begins to slip, the others act to balance everything which reduces the speed of the slipping motor almost 'by magic', but maintains it at a point just below slip. Recovery of wheel slip (providing the driver is not in-attentive or deliberately attempting to slip) is very rapid. It is this system that allowed the incredible acceleration for which class 74 was known.
The power controller allowed infinite variability but standard notches were provided so class 74 could work with its smaller sister, class 73/1, or with any standard Type 3 diesel locomotive. Two constant voltage notches were provided for slow speed control of shunting and coupling-up. Three further notches spaced out over the power range corresponded to the 'Series, Parallel, Weak-Field' (2, 3 & 4) settings of power controllers in multiple unit stock.
Control equipment was based around the 'Line Replaceable Unit' (LRU) ethos and consisted of 'trays' of circuit boards and equipment arranged in two cabinets. No.1 housed just the usual circuit-breakers, relays, reversers and so forth. No.2 also housed standard equipment but included the complex electronic control circuits that ultimately were the un-doing of the class. Each tray could be slid in and out of the cabinet (with the locomotive switched off) with ease and each had a test connector. Specific testers were made that simply compared the electrical signals and stimuli on the test socket with the design values. If any were not as expected, the whole tray was replaced and the locomotive was good-to-go... at least that was the theory. The use of testers plugged into the test socket removed the need for engineering staff to be competent electronic engineers (as opposed to electrical engineers which they undoubtedly were), but often faults on one tray would only show themselves in combination with faults on other trays. This lead to confusing diagnostics and locomotives were often failed for long periods pending in-depth exploration of a fault. Faulty trays were sent for repair. This early attempt was bold and fundamentally sound. Electronic circuit arrangements in modern locomotives use this approach almost unchanged. Unfortunately, powerful control and computational electronics were in their infancy in 1968. Had the resources of just ten years later been available, it is arguable that class 74 would have been a success.
Complications with rebuild
The body of HA/class 71 was never intended to carry any weight, the construction following typical methods for the 1950s of massive under-frames with the body perched on top to stop everything getting wet. In August 1966, after initial strip-down and examination of E5016, engineers were forced to re-assess the build when it became apparent the planned equipment changes could not be accommodated. It was even mooted that the body would have to be divided and lengthened. The additional weight (even after dispensing with the flywheel and other heavy equipment) meant the body had to be re-constructed and stressed by use of Warren girder framework with outriggers to support the curved body skin. Translucent roof panels were fitted to increase daytime illumination in the engine room - to great effect.
One might think putting a diesel engine into an electric locomotive with a booster set is simply a matter of coupling the crankshaft of the engine to the main shaft of the existing booster through some clutch/gearbox arrangement, but this was not so. When working on diesel power, the engine drove generator EE843/1C (615 V output) direct to the booster power input. It must not, however, be thought that the 615 V input was a direct replacement of the conventional electric supply - the configuration of the booster set did not work simply by utilizing the line voltage. See the article on Boosters for clarification. The equipment train of Engine+Generator+Booster proved too long with the existing equipment layout. Modifications were made (cooler groups, water tanks and silencer were mounted in roof section over the equipment room) and very little of the progenitor locomotive layout remained. All was well with production back on track.
The Paxman engine of class 74 was marginally more powerful than class 73's English Electric engine but it was nowhere near as reliable. The Paxman was also noisy (due to the silencer mounted in the roof very near the cab) and difficult to start on occasion - rather negating the novel change-over whilst in motion and subsequent flexibility off the third rail. The class's generally poor reliability often led to (unscheduled) electric to diesel changeovers when on the move; regular passengers on Class 74-hauled passenger trains became quite used to this. Until four additional '4-REP' units were built in 1973/74, class 74 had regular daytime passenger turns including the 15:30 Waterloo-Weymouth. Subsequently their only regular passenger runs were the night mail & newspaper runs to/from Bournemouth and boat trains (which were not available to regular ticket holders).
In practice, class 74 was a rare visitor to Weymouth (because failures on diesel were common and a stranded train, irate passengers and blocked line are highly undesirable) and was usually replaced at Bournemouth by the redoubtable class 33 for the final leg of the journey. This was almost directly attributable to their problems when running on diesel power and totally negated their "raison d'être". As a result, they failed to deliver the expected advantages of a higher power (than class 73) electro-diesel. Services to Southampton Ocean terminal did use them frequently over non-electrified lines, but this required only a few miles of diesel haulage rather than 60+ on the trip to Weymouth and back. Also, following the closure of Southampton Terminus Station, the section of non-electrified track served Ocean Terminal exclusively - a failure here would not inconvenience other services - the same was not true West of Bournemouth. The steep ascent from Weymouth would also have taxed their power output to the utmost, as the 'Channel Islands Boat Train' usually loaded to 11 cars.
Class 73 had a simpler electrical system arrangement for control of the dual power sources - even to the point of two separate power controllers on the driver's desk; one for diesel and one for electric. The complex control system of class 74 (with one dual-purpose controller) was problematic, and the class was dogged with electrical system failures up to their last days. Together, the three complaints of poor reliability, difficult engine starting and excessive noise made class 74 unpopular with crew and fitters alike.
When in good order they were sprightly performers and running on third rail with the full 2,552 hp brought to bear (the original class 71 traction equipment having been down-rated slightly from 2,700 hp to extend service intervals), many 100+ mph runs were noted despite a stated maximum of 90 mph - though nothing approaching this was achievable on diesel power. Their electronic power delivery meant that acceleration was equally impressive on both diesel and electric power sources. Running on the diesel engine however, available power was pretty much exhausted by the time 60 or 70 mph was reached with 'normal' loads and heavy loads (even 8 or nine loaded milk tanks over the banks of the West London Line) would tax them. Speeds would drop and recovery margins in timings taken full advantage of.
They were regular visitors to the London area, often running on Midland and Western Region tracks. In the early half of the 1970s, milk trains for the Southern Region were a staple for the class as far as Acton Yard, requiring diesel power from Clapham Junction through Kensington and onto the Western Region mainline (class 52 or occasionally 47 were used for the major haul west of Acton to & from Cornwall and Devon). This would often produce two trains per weekday and usually two different examples of the class.
Although their multiple working abilities allowed them to control (with a single crew) each other, class 73, any EP multiple unit stock and any blue star coded type 3 diesel locomotive, multiple operations were exceptionally rare. Wherever class 74 was noted with another locomotive, the 74 would invariably be 'dead in-train' - i.e. failed and subsequently rescued. One exception to this were the frequent balancing runs between their home depot of Eastleigh and their day-time out-stations of either Clapham Junction yard or Stewarts Lane depot in readiness for night-time postal and newspaper trains from Waterloo. These were nearly always light-engine or occasionally combined with empty coaching stock (ECS) moves with both locomotives under power - over 5,000 hp.
The entire class was allocated to
Eastleighdepot for their operational life, and 74003 was the last locomotive to enter Eastleigh works for repairs. In common with class 73, class 74 were sent to Crewe for heavy maintenance up until 1972, when Eastleigh took over all treatment of both classes.
In 1976, the first example was withdrawn: 74006 (erstwhile E6106) was damaged by fire and beyond economical repair.
Although at this time, the electronic control problems of the class could have been resolved (by virtue of the evolution of electronics) and British Rail had demonstrated a willingness to undertake heavy re-design work on other locomotive types to improve reliability (notably on classes 82, 83 & 84) the work for which the class had been built was drying up. Boat trains were greatly reduced in number, and many of the remaining were replaced with multiple units. Freight and parcel work was contracting and class 74 was embarrassingly short of work (as was the progenitor class 71), due purely to the changing tides and fortunes of the railway business. In July 1977, 74002 was withdrawn following a collision and a month later, 74009 failed and budgetary permission to repair was not forthcoming. On the last day of 1977 the seven remaining members of class 74 (and all 14 members of class 71) were withdrawn from traffic. They languished at Eastleigh depot for almost a year until all but 74010 were stripped of all reusable components and finally dispatched to various scrap yards. 74004 & 74009 remained at Long Marston until August 1978 whence they were reduced to scrap. 74005 was the last surviving member of the class being finally broken-up in January 1981 at
Frattonnear Portsmouthnot far from its home depot.
As other members of the class were being put to the cutters torch, Eastleigh performed a light overhaul on 74010 and returned it to fully servicable condition. In 1978, it was towed to
Derbydepot for evaluation as a potential candidate for departmental use at the Railway Technical Centreand remained in the depot yard for some time. Despite being in fully working order with a mass of spares available (from sister locomotives now scrapped) it was eventually deemed unsuitable, towed to Doncasterand scrapped in 1979.
* [http://www.semgonline.com/electric/class74.html SEMG site]
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