Number Series 25001 – 25025 25026 – 25082 25083 – 25149, 25150 - 25247 25248 - 25327
Original Numbers D5151 – D5175 D5176 – D5232 D5233 – D5299, D7500 - D7597 D7598 – D7677
Class 25 / 0 25 / 1 25 / 2 25 / 3
Diagram Number 25 AV 25 BV 251 AV 251 BV 251 CX 251 DX 252 AV 252 BV 252 CX 252 DV 252 EX 252 FX 253 AV 253 BX
Type vo vo vb vo xb xo vb vo xb vb xo xo vo xo
Weight (imperial) 70ton 5cwt 72ton 7cwt 73ton 15cwt 71ton 9cwt 74ton 8cwt 72ton 3cwt 73ton 1cwt 70ton 14cwt 74ton 8cwt 73ton 15cwt 70ton 12cwt 70ton 0cwt 70ton 14cwt 70ton 12cwt
(metric) T 71.4 73.5 74.9 72.6 75.6 73.3 74.2 71.8 75.6 74.9 71.7 71.1 71.8 71.7
Maximum Axle Load 18ton 17cwt 18ton 15cwt 18ton 5cwt     18ton 12cwt 18ton 1cwt   18ton 15cwt     18ton 1cwt  
Train heating boiler type (Stone Vapor) N/A L4610 N/A L4610 N/A L4610 N/A L4610 N/A N/A
Heating Water Capacity, gal N/A 580 N/A 580 N/A 580 N/A 580 N/A N/A
Fuel Capacity, gal 520 620 510 510 510
Generator Type BTH RTB15656 BTH RTB15656 BTH RTB15656 BTH RTB15656
Generator rating (continuous) 817kW, 750V at 1,090A 819kW, 780V at 1,050A 819kW, 780V at 1,050A 819kW, 910V at 900A
(1 hour) 545V at 1,500A 545V at 1,500A 545V at 1,500A 545V at 1,500A
(maximum) 2,525A 2,640A 2,640A 2,640A
Auxiliary Generator   AEI RTB 7440 continuous rating 50kW AEI RTB 7440 continuous rating 50kW AEI RTB 7440 cont. rating 54kW
Traction motors (4) type BTH 137BX rated 375A at 545V AEI 253AY rated 650A at 315V AEI 253AY rated 650A at 315V AEI 253AY rated 650A at 315V
Traction motor gear ratio 79 : 18 67 : 18 67 : 18 67 : 18
Rail hp 949 949 949 949
Maximum tractive effort, lb 39,000 46,500 46,500 41,500
Continuous tractive effort, lb/mph 20,800/17.1 20,800/17.1 20,800/17.1 20,800/17.1
Engine Sulzer 6LDA28-B pressure charged four stroke
Continuous rating 1,250hp at 750 rpm
Maximum speed 90 mph
Wheel Arrangement Bo - Bo
Length over buffers 50ft 6in
Width over body 9ft 13/8in Note manufacturer quotes 8ft 115/8in for class 25/1 onwards  
Height 12ft 8in
Total wheelbase 36ft 6in
Bogie wheelbase 8ft 6in
Bogie pivot centres 28ft 0in
Wheel diameter 3ft 9in (45 in)
Minimum curvature 41/2 ch. (297 ft.) rad.                
Lubricating oil capacity 100 gal
Coolant 186 gal
Route availability 5
Coupling restriction Blue Star                

As you might expect when a number of locomotives are built over a six year period a lot of design changes and minor modifications will occur during that time.  I have attempted to record those changes and provide details of the significant characteristics of these locomotives.  Unfortunately not all sources of information agree with what these actually were!  At the end of this section you will find a list of the sources used to compile this table and where I think there may be discrepancies.  Should you be able to correct anything you read here then I would be pleased to hear from you.


The British Railway Modernisation plan announced on 25 January 1955 is generally considered to be the turning point in the change from steam to diesel and electric motive power.  The main driver behind this was to reduce operating costs through a rapid replacement of steam by diesel traction.  On the 16 October 1955 a pilot scheme to purchase a total of 171 main line diesel locomotives (that soon became 174) was approved of which 106 were designated mixed traffic locomotives with a power range between 1,000 and 1,250hp.  Of these 20 were of the BR Derby design that became the first of the class 24 locomotives, the precursor of the class 25 design.  Eventually 151 class 24 locomotives were constructed but after the delivery of the first few units it became apparent that the speed ceiling of 75mph was unduly restrictive and the provision of a bit more power would be advantageous.  In the course of normal development the power output of the Sulzer six-cylinder engine had been increased to give a continuous traction output of 1,250bhp at 750rpm by the introduction of charge air cooling and the first locomotives to use this became known as class 25 locomotives.

Class 25/0

The Class 25/0 locos were the first Sulzer Type 2's to be built using the newer 1,250hp "B" engine, modified generator assembly and traction motors. This increase in horsepower was obtained from an air/water free flow intercooler fitted between a higher capacity pressure charger and inlet manifold, included within the normal cooling circuit to maintain simplicity. The cylinder head was also modified and strengthened.

The BTH generator, type RTB 15656, was rated as 817.5kW, 750/545V, 1090/1500A at 750 rpm, only slightly different from the earlier Class 24's.  (Note all Class 25 locomotives used a generator also designated as BTH RTB 15656 but its rating and characteristics changed over time).  The generator supplied four BTH 137BX traction motors connected in parallel and rated 245hp, 545V, 375A at 560 rpm with a gear ratio of 18:79 (to give a 90mph maximum speed).  Maximum tractive effort was 39,000lb whilst continuous t.e. was 20,800lb at 17.1mph, the latter standard for all Class 25's.  At rail hp was 949, now available between 9.3 and 77.6 mph. The first fifteen locomotives fuel capacity was 520 gallons (design type 25 AV) whilst the final ten had larger 620 gallon fuel tanks installed (design type 25 BV).

BT-H (British Thompson-Houston) merged with Metropolitan-Vickers to form AEI (Associated Electrical Industries) on the 1st January 1960.  AEI was subsequently absorbed into GEC in 1967.

Class 25/1

The Class 25/1 featured the new AEI 253AY traction motor, the motors were a result of the collaboration between BTH, MV and American builder Alco. This smaller, lighter motor was an attempt to market a traction motor to a worldwide audience, especially to the meter gauge lines. For Class 25 locomotives these lighter motors meant the discontinuance of other weight saving measures being built into the design. They would be highly rated, being an attempt to overcome the loss of tractive effort normally found on starting, the field divert system was also modified to allow increased capability throughout all the speed ranges.

The main generator was a 12-pole machine with the rating changed to 819kW, 780/545V, 1050/1500A at 750 rpm.  (Note I have also seen the continuous rating quoted as 819kW, 630V, 1300A).  The four traction motors were now connected as series parallel pairs being rated at 234hp, 315V, 650A at 460rpm, with a gear ratio 18:67.  Pairs of motors connected in series provided a higher maximum tractive effort (usually quoted as 45,000lb although 47,000lb could be achieved) but the downside being that a series pair connected machine was more prone to slipping that one with an all parallel grouping.  Full power was available between 7 and 77.5 mph, an improvement over Class 25/0 locomotives with all other ratings unchanged from the earlier series. The traction motor’s continuous rating of 650 amps was not far removed from its one hour short term or 'emergency' rating of 680 amps, and this could only be monitored manually.  On heavy trains close monitoring of the ammeters was necessary to avoid motor damage.  Whilst the body shell remained similar to D5151 there were a number of refinements.  Relocated were the air horns to either side of the headcode panel.  The cab skirt and body fairing were discontinued, though the support lugs remained.  A new driving control panel was fitted.  The fuel & water tanks were also redesigned with a fuel capacity of 510 gallons.  (Note I have also seen this also quoted as being 500 or 560 gallons).

There were initially two variants of this sub-class.  The vast majority were boilered and designated 251 AV.  The four without train heating were designated 251 BV.  In due course, when it was decided to fit dual braking to a number of locomotives, those previously 251 AV became 251 CX and one of the 251 BV (25032) became 251 DX.

Class 25/2

The Class 25/2’s featured re-styled body work and two tone green livery similar to that carried by the Brush Type 4's (class 47's). The redesign principally affected two areas, the cab and the location of the air intakes. The gangway doors fitted to the earlier examples were rarely used, their presence certainly added to the complaints of noise and draughts in the cabs. The removal of the air filters from the side air louvers to the cantrail was the result of a comparison carried out at Inverness between a batch of Derby built Type 2's and a batch of BRCW Type 2's, the tests targeting the air quality within the engine room.  These tests revealed the location of the grilles on the Derby build allowed for much more debris to reach the filters (especially the lower ones), clogging them quicker, leading to poorer air quality within the engine compartment, and so potentially affecting performance and engine wear. With such a large order to be completed it was felt that a redesign of these areas would have a cost savings in the long run, in addition to a better working environment within the cabs, and with a general less cluttered look about the locomotive's exterior.

In 1962 Sulzer designed and began development of a prototype engine for higher outputs based on the LDA range.  Rated initially at 1,700hp at 850rpm (with a development potential to 2,000bhp at 850rpm) it was approximately the same overall size as the 6LDA28 and designated LDA28-R.  BR were approached with the idea that one of the Derby Type 2s should be fitted with this engine and development work proceeded slowly until problems with the 12LDA28-C (used on the class 47 locomotive) diverted resources.  In the end development was terminated and the locomotive set aside for its use, D5299, was completed as a standard class 25/2.

There were six variants of this sub-class, reflecting that locos were boilered and/or vacuum braked and/or dual braked.  Boiler fitted locomotives included the first five (252 AV) and final thirty class 25/2 (252 DV).  Only members of the latter batch were modified for dual brake operation becoming 252 CX with the exception of 25242 that had had its boiler removed and was designated 252 FX.  The non-boilered vacuum braked locos were 252 BV and when dual braked became 252 EX. 

Class 25/3

Whilst these locomotives still carried a RTB 15656 generator this variant was a ten pole machine with a modified assembly incompatible with earlier equipment. The regulated (full hp) part of its characteristic was substantially the same as before but the unloading point, that is the point at which full power could no longer be utilised, was altered to 900A, 910V (819kW) from 1,050A, 780V (819kW).  Only two stages of field weakening were employed, previous machines had six, and this provided ‘full power’ at speeds between 7 and 80 mph, whilst maximum tractive effort was reduced to 41,500 lb. 

The latter half of the 1960's had seen the widespread introduction of solid state electronics and these locomotives incorporated a control system where speed was detected electronically rather than mechanically.  A signal from a tachogenerator was used to close contactors in sequence at given speeds to activate the motor’s field weakening process, rather than through contacts and relays as in earlier types.  The control system ensured the traction motors an main generator were all operated within the continuous rating of the machines except in full field conditions when the driver was able to judge how long to remain in the short-term rating condition.

Whilst all the other class 25 locomotives had been built at either Darlington or Derby workshops, the final Type 2 order were to be built by Beyer Peacock of Manchester.  However, because of financial problems the final 18 locomotives were transferred to BR Derby for construction.

There were two variants of the Class 25/3 sub-class.  253 AV locomotives were fitted with vacuum brakes and in due course many of these were dual braked and redesignated 253 BX.  By the time the last few locomotives were under construction dual braking had become the norm and ten of the last batch from Derby were built new as 253 BX locomotives for work out of Willesden on the recently upgraded West Coast Main Line.

Class 25/9

The twelve locos that formed this sub-class were all Class 25/3 locomotives and the intention was that they would operate on traffic won for the Industrial Minerals Division of Railfreight that included salt for road gritting from the ICI mine at Winsford.  The locos were selected from the available pool of Class 25 locomotives in March 1985 with the expectation of three more years of service before 10,000 running hours since last Works attention would be reached and their maintenance being concentrated on Carlisle Kingmoor depot.  At that point the expected cascade of motive power on BR as a whole would see them replaced by Class 31 locomotives.  However by the traffic they were designated for was not captured and in due course the sub-class were withdrawn along with the other members.

Note: The sources used to compile this information are:

1)       AEI/Sulzer (1962) “Operating Manual for BR Type 2 Diesel Electric Locomotive Numbers D5000 – D5150 (1160hp) and D5151 – D5175 (1250hp)”, AEI/Sulzer Ltd

2)       AEI/Sulzer (1966) “Operating Manual for BR Type 2 Diesel Electric Locomotive Numbers D5176 – D5299 and D7500 – D7677”, AEI/Sulzer Ltd

3)       Webb, B (1978) “Sulzer Diesel Locomotives of British Rail”, David & Charles

4)       Tayler, A.T.H (1984) “BR Locomotives:2 Sulzer Types 2 and 3”, Ian Allan – in my opinion the definitive guide to the construction and operation of class 25 locomotives

5)       Marsden, C.J (1985) “Brush-Sulzer Locomotives”, Ian Allan

6)       BR/Sulzer Type 2 website

7)       Rail Express Magazine, Issues 1 and 2, June/July 1996

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