The following notes provide a summary of the prototype variants.
The Class 24s and 25s are complex classes and we can only give a limited amount of information in these notes. There were variations in the batches when built and they were then subject to a number of modifications during their operational career. For more detailed information we would suggest the book “Diesels in Depth the Class 24/25’s” published by Ian Allan in 2006 and studying the photo database www.rail-online.co.uk where there are nearly 150 photos of class 24s and 200 of class 25’s throughout their operating life. The great feature of this site is you can search by class/period etc and using the F11 function view the photos in full screen mode. We would advise being cautious to copy a preserved example as these can represent the “final” operating condition and can demonstrate plumbing runs and modifications that were not in use during their BR service.
Head Code Boxes
Class 24/1 The first fifty Class 24s were built with head code discs to indicate the class of train worked, but from D5050 to D5150 (later renumbered 24050 to 24159) they were fitted with Train Indicator Headcode boxes. This was to display the new style train reporting numbers using a roller blind system as used on buses but using a system of numbers and letters to indicate the type of train and its destination. These first indicator boxes had plain sides because the locomotives retained the air horns behind the buffer beam. The indicator box was not a separate add-on to the roof but was integral with the cab structure so when subsequently indicator boxes changed to individual locomotives this was the result of a complete cab swap.
Class 25/0 D5151 to D5175 (25001 to 25025) retained the same body style as the 24/1 and valances and were also fitted with the same headcode boxes. The reason for the different classification was a change in the engines rather than any external changes. This batch of 25 locomotives were intended for freight traffic and was therefore not fitted with train heat boilers and associated water tanks.
Class 24/1 and 25/0 with modified Head code boxes
With the building of the Class 25/1 and 25/2 the headcode boxes were revised with sloping side extensions which also incorporated 2 circular housings for the air horns. However a small number of the Class 24/1 and 25/0 which had been built with the head code boxes with vertical sides received head code boxes with the sloping side extensions but without the air horn housings, sometimes at one end only, sometimes at both ends.
5127 modified head box one end only.
24 133 modified head code box one end only (circa 1976)
24 134 modified head code box both ends but one end also had the air horn housings.
D5147 (24 147) seen in 1968 in green with full yellow ends with modified head code boxes at both ends. The loco retained these head code boxes when re painted as 24 147 in blue and the valances removed.
5148 (24 148) seen in 1970 in blue with modified head code boxes at both ends.
25 009, modified head box one end only (circa 1977).
Boiler Room Grill Covers
All the locomotives built were fitted with an air intake to provide air for the steam heat boiler even though half of the class was built without the train heat boiler and its associated equipment. This air intake consisted of a grill panel two thirds of the way up the body side, but experience with members of the class operating in the Highlands drawing in excessive quantities of cold air into the boiler compartment led to the development of a blanking plate to cover the grill, which remained underneath. Subsequently this blanking plate was modified with five slots to allow a lower volume of air into the train heat boiler compartment. However on at least one loco the slotted cover had it replaced with a solid one!
Examples seen with 5 slots include
24 117 (with tablet catcher recess)
D5118 (with tablet catcher in 1970)
5119 (seen in 1970)
5121 (seen in 1974)
D5123 (late 1960’s)
5125 (seen in 1971)
5126 (seen in 1971)
24 132 (with tablet catcher recess)
24 115 (in 1976)
24 118 and 24 123 (in May 1974)
Examples with 4 slots include
D5127 was seen with a plate with 4 slots in it in 1966 and 1974 (as 5127 in blue and as 24127 in October).
The solid blanking plates appeared on most of the class, even those originally fitted with a train heat boiler. Many locomotives originally fitted with train heat boilers had the boilers removed and those that retained the fitment rarely found themselves on a passenger train that required a working boiler.
Circular Exhaust Ports
When built the class 24s were fitted with a silencer and circular exhaust ports in the roof. Following problems with fires in the silencers the silencers were removed and the exhaust went strait through the roof above the turbo charger. At the same time the exhaust port changed to a rectangular shape and was located further towards the middle of the locomotive. The first locomotives modified were seen in 1963 and subsequently all locomotives were modified, but in some cases this took some years to complete and locos could still be seen with the circular ports into the early 1970’s. Even after the exhaust design had changed new locomotives continued to be built with the circular exhaust ports before they moved to the revised design. As an example some of the Class 25/2 and 25/3 (the revised body design) were built new in the 1960s with circular exhaust ports.
Fuel and Water Tanks
Through out their lives the style, capacity and positioning of the fuel and water tanks was changed usually in an attempt to keep the overall weight of the locomotive down without affecting their operating efficiency and range. The first batch built were all considerably overweight and for their life the class were subject to various schemes to reduce the weight.
As with many changes to the class some of the modifications applied to new batches were subsequently applied to previous members of the class but there was little consistency in this process.
The various styles of fuel and water tank tanks are as follows:
1. Class 24. (D5000-49, 24001 – 24049 as built). In these examples the fuel and water tanks were wrapped around the two centrally located battery boxes with the break between the fuel and water tank (under the batteries) being off centre (the fuel tank being longer).
2. Class 24/1. (D5050-120, 24050-120). The first attempt at reducing the weight came with this batch of locomotives when the vertical section of the fuel tank was reduced, no longer reaching up to the frames.
3. Class 24/1. (D5121-50, 24121-50). For this batch the design of the water tank was reduced with the horizontal section reduced in length creating a gap between the fuel and water tanks. Subsequently many of this batch had gauges fitted to both the fuel and water tanks, or sometimes just to one of the tanks.
4. Class 25/0. (D5151-75, 25001-25) The Class 25’s started where the Class 24 left off with a fuel tank that did not reach up to the frames on the vertical surface.
5. Class 25/1, 25/2 and 25/3. (D5176 onwards, 25026-327). The first of the 25/1s received the final variation with a completely new arrangement of fuel and water tanks. In this case the battery boxes were also revised to a single tray per side (formerly 2 trays per side) and the fuel tanks located underneath. This is what we refer to as the “balanced fuel tank”. The separate water tank (where fitted) was more rectangular and located to one side (at the boiler grill side). As with the earlier examples some were fitted with gauges and some not.
Once the various batches (Class 24, 24/1, Class 25/0) had been delivered they were subsequently modified so locomotives fitted with the original fuel or water tanks could be fitted with some of the shortened water tanks and reduced height fuel tanks (or both), very much a mix and match arrangement. To model a particular locomotive the advice is to find a photo at the required period and identify the required modifications. When a locomotive entered works the locomotive would be completely stripped down with all the removed components sent for refurbishment to the various sections in the works and during the re building process the first available component would be fitted hence locomotives exchanging cabs, sandboxes and fuel and water tanks.
Finally, to ease access to the pull out batteries and other components, the body side bottom valencing was cut back from the majority of the locos from the late 1960s onwards; indeed many 25/2’s were delivered without this feature.
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