The final batch of locomotives were designated Class 25/3 and built by Derby Works and Beyer, Peacock and Company of Manchester.

The Class 25s were primarily designed for freight work, but a significant number were fitted with boilers for heating passenger trains.

[3] The first 25 locos became known as Class 25/0 and were built at the BR Darlington works using the newer 1,250 hp (930 kW) "B" engine, modified generator assembly and traction motors.

This increase in power 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 generator supplied four BTH 137BX traction motors connected in parallel and rated 245 hp (183 kW), 545 V, 375 A at 560 rpm with a gear ratio of 18:79 (to give a 90 mph (140 km/h) maximum speed).

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.

With such a large order to be completed it was felt that a redesign of these areas would have 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.

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 final batch of locomotives were designated Class 25/3 and was to be built by BR's Derby Works and Beyer, Peacock and Company of Manchester.

However, because of financial problems Beyer, Peacock was unable to complete the final 18 locomotives and these were transferred to BR Derby for construction.

Though these locomotives still carried a RTB 15656 generator, this variant was a ten pole machine with a modified assembly incompatible with earlier equipment.

Only two stages of field weakening were employed, previous machines had six, and this provided ‘full power' at speeds between 7 and 80 mph (130 km/h), and maximum tractive effort was reduced to 41,500 lbf (185,000 N).

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.

At the end of 1985 twelve of the remaining Class 25/3 locomotives were designated as 25/9, the intention being 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 would be concentrated at Carlisle Kingmoor TMD.

In 1962 Sulzer designed and began development of a prototype diesel engine for higher outputs based on the LDA range.

Rated initially at 1,700 hp (1,300 kW) at 850 rpm (with a development potential to 2,000 bhp (1,500 kW) at 850 rpm) it was approximately the same overall size as the 6LDA28 and designated LDA28-R. BR was approached with the idea that one of the Derby Type 2s should be fitted with this engine but development work proceeded slowly and problems with the 12LDA28-C (used on the Class 47 locomotive) diverted resources.

It was then restored by apprentices at Leeds Holbeck shed including repainting in BR two-tone green and having its original number of D7672 applied.

[16] The class returned to the main line in October 2007 when D7628 (25278) worked from the North Yorkshire Moors Railway to and from Whitby station.