Engine

The main power for the class 24 was the Sulzer 6LDA28 diesel engine - denoting 6 cylinders; Locomotive use; Direct fuel injection; (turbo-charged); 28 cm (11 in) bore cylinders. This was effectively an off-the-shelf purchase with small changes to bearings, injectors and some other minor items. The same engine was used in the CIE 101 Class locomotives in Ireland.

Transmission

The diesel engine powered another off-the-shelf product, the British Thomson-Houston (BTH) RTB15656 main generator which, in the class 24, was rated at 735 kW (986 hp), 750/525 V and 980/1400 A at 750 rpm. Traction motors, one per axle, were also by BTH being the type 137BY rated at 222 hp (166 kW), 525 V, 350 A at 560 rpm connected to the axle via a 16:81 gear stepdown ratio, each force ventilated by an AEI 12.2 hp (9.1 kW) electric motor.

Train heating

The original pilot scheme locos (D5000–D5019) were fitted with a Stone Vapor type OK4646A steam heating boiler with a 600 imperial gallons (2,700 litres; 720 US gallons) water tank. The following ten locos had the similar 1,750 pounds (790 kg) per hour type OK4616B and a reduced water capacity of 450 imperial gallons (2,000 litres; 540 US gallons), and this was perpetuated in the remaining production run which used the Stone Vapour 1,000 pounds (450 kg) per hour type L4610 boiler. These variations meant that the initial batch of 20 locomotives tipped the scales at 79 long tons 16 hundredweight (81.1 t; 89.4 short tons); the following 10 locomotives slightly lower at 78 long tons 16 hundredweight (80.1 t; 88.3 short tons); the remainder of the Class 24/0 at 77 long tons (78 t; 86 short tons); and the Class 24/1 at 73 long tons (74 t; 82 short tons) exactly. During subsequent years the boilers were removed from the majority of Class 24/1 locomotives, reducing the overall weight by 2 long tons (2.03 t; 2.24 short tons). Ten locomotives (D5102–5111) had no train heating, the space being occupied by the air compressors needed for operation of the Consett iron ore trains.^[2]

Other systems

Several of the systems within the class 24s were standard. The braking system was the standard BR system, adopted as part of the Modernisation Plan, of locomotive air and train vacuum brake, both applied by a single handle via a proportional valve. Similarly, the connection for multiple working was the standard electro-pneumatic system designated "Blue Star" with each loco bearing a small blue coloured 5-pointed star above each buffer to denote this. Also common was the provision of a door to allow staff to pass between locomotives, or between a locomotive and adjacent coach. In practice, these were rarely used and were sealed shut at overhaul during the 1970s to reduce draughts in the cab.

With production reaching 151 there were some differences between batches of locos too. Ten of the initial twenty had "Athermos" pressure-lubricated plain bearing axleboxes rather than the more usual roller-bearing axleboxes. Although these remained for the life of the locos they were the only ones so fitted. Much more noticeable were D5114–D5132 which were fitted with tablet catchers on the side of the driver's cab for use on the Far North Line from Inverness. Also very visible were the roof-mounted headcode boxes fitted from D5114 giving an outward appearance very similar to the later Class 25 but without horn grilles.

Liveries

The pilot scheme locomotives were delivered in overall green livery with a grey roof and black below the body. D5000 was delivered with a narrow light grey stripe at waist level while the remainder had a broad light grey stripe at solebar level.^[3] This light grey stripe may have appeared bluish and has been described as pale blue or eggshell,^[4] but in most illustrations, it appears to be off-white. At first, green liveried locos had plain green ends, but this was changed from 1962 to small yellow warning panels, and then from 1967 to full yellow ends, some locos receiving these while still in green livery. At least five locos were repainted in two-tone green livery (applied along with the small yellow warning panel) in a similar manner to Class 47s and some Class 25s.^[4][5]

The first class 24 painted in Rail Blue livery was D5068 in December 1966, and repainting continued into the mid-1970s, although some locomotives were withdrawn before being painted in this colour.

Alterations

As with many large classes of locomotive, there were some variations during the Class 24s' lifetime, some affecting all of the class (nominally), and others just individual locos. One such locomotive was 24133, one of the last survivors of the class and easily recognised as it had different headcode boxes on each end, the standard one for the class at one end and that at the other end matching those used on the Class 27.^[6] This was not unique, as 24145 had a headcode box similar to those fitted to later batches of Class 25s.^[7] In each case, this is likely to be due to collision damage repairs. Some Inverness-based locomotives had 'car lights' fitted to each nose. These were after-market car headlamps mounted in the plated-over nose doors, to provide extra visibility to users of level crossings on sharply curved branch lines. D5114-132 had pairs mounted in each nose. These were removed between 1975/6, with small plates welded over the apertures. 12 Class 26s had similar modifications, some having pairs and others having a single lamp. Examples of Class 37s, Class 47s and a solitary Class 86 had single railway spotlights installed at each end but the class 24s and class 26s were the only ones fitted with proper car headlamps.^{[citation needed]}

In 1960 D5008 was fitted with a Pressure-charging Protection Unit which was designed to prevent the engine from running continuously above the smoke limit. The unit failed on one occasion allowing significant smoke emission, and in general did not demonstrate any significant impact on the amount of visible smoke emitted. The unit was subsequently removed.^[8]

Departmental use

In November 1975, 24061 was transferred to BRs research department based at Derby Works, and was allocated the number RDB968007.^[12] This loco was subsequently renumbered as 97201 and was finally withdrawn on 4 December 1987,^{[citation needed]} the last Class 24 to be operational on BR. After withdrawal in 1976 two Class 24s were converted into train pre-heating units and were allocated to the Western Region: 24054 was withdrawn in August^[12] and became TDB968008, being allocated to Newton Abbotuntil withdrawn in October 1982;^[12] 24142 became TDB968009 and was allocated to Landore.^[16][17][18]

Withdrawal

The very first Class 24 to be withdrawn was in November 1967 when a fire broke out on D5051 while it was working a train of empty coal wagons in Scotland. The damage was too severe for it to be repaired, and it was cut up at Inverurie in August 1968.^[19][20] It had lasted exactly 8 years. The second Class 24 to be withdrawn was also as the result of an accident in Scotland, this time at Castlecary. In this case D5122, running light engine, hit a stationary DMU at about 40 mph (64 km/h), the impact and subsequent fire bending the main frames and completely destroying the No.2 end. The accident happened on 9 September 1968 but despite being withdrawn the same month, the locomotive was not finally cut up until March 1971.^[20][21] A total of fourteen Class 24s were withdrawn and scrapped without receiving a TOPS number.^[20]

Some Class 24s were withdrawn in 1973 with the closure of the Waverley route, which linked Carlisle and Edinburgh, but most of these were put into storage and subsequently re-entered traffic to fill gaps left by the movement of Class 25s to cover the withdrawal of Class 22s on the Western Region. Thus Class 24 withdrawals only started in earnest with completion of the Glasgow electrification in 1974, and re-allocation of Class 26 and Class 27 locos saw the Class 24s concentrated around various Lancashire depots and at Carlisle. On 27 November 1976 there were just ten Class 24s still in operational service, all allocated to Crewe Diesel Depot (depot code CD).^[22] However, with the reinstatement of 24082 and 24073 this number climbed to 12 by February 1977.^[23]

In January 1978, two farewell railtours were run – 24082 and 24087 hauled the Merseyside Express from London St Pancras to Liverpool and return on the 14th,^[24] and 24087 and 24133 hauled the Cambrian Coast Express from Birmingham to Aberystwyth and Barmouth on the 28th. On the latter trip, 24087 failed on the outward journey, was dumped at Machynlleth on the return journey and never worked again.^[25]

On 21 January, 24133 had also taken part in the "Farewell to the 44s" tour, providing steam heating for the coaches while 44008 Penyghent provided the motive power, on the Crewe to Chester leg of a circular tour from London.^[26]

For the summer of 1978, six class 24s remained in service – 24023, 035, 047, 063, 081 and 082.^[23] In May, a North Wales DMU passenger diagram was converted to loco haulage on Mondays to Fridays, comprising the 09:42 Llandudno - Manchester, 13:30 return, 16:42 Llandudno - Crewe, 20:30 Crewe - Bangor and 22.45 Bangor - Llandudno Junction. At least five different class 24s were recorded on this diagram during the summer.^[27] Additionally, on Saturday 10 June, 24082 worked a Llandudno Junction – London Euston relief train all the way to London due to a lack of a replacement loco at Crewe.^[28]

By January 1979, only three locos remained in use – 24063, 081 and 082. 24082 was withdrawn on 1 March and 24063 on 9 April, leaving 24081 the last in service.^[23] The final recorded passenger duty of a Class 24 was on 2 August 1979 when 24081 rescued 40129 at Colwyn Bay on the 18:05 Holyhead – Euston, hauling the train as far as Crewe.^[29]

Cutting up of some Class 24s was carried out at Swindon Works on the Western Region, an area which had never received an allocation of Class 24s. The first locos were 24042, 24045, 24048 and 24050 which were moved from Derby Works in December 1975,^[13] and the last of the 67 Class 24s cut up at Swindon was 24084 in early December 1978.^[20][27][30]

The very last Class 24 to be withdrawn from operational service was 24081.^[12][31] This loco, allocated to Crewe Diesel Depot had been considered something of a celebrity, lasting over a year after the previous withdrawal of 24063 on 9 April 1979.^[31] 24081 was finally withdrawn in October 1980 having worked its last revenue earning train, the 05.43 Grange - Shotwick freight on 7 January 1980, and then making guest appearances at Nuneaton, Crewe Works Open Day, and Southport.^[31]