The 660 & 760 Class railcars are a Diesel Multiple Unit train built by the New South Wales Government Railways (NSWGR) and used in the latter stages of their life by CityRail, primarily on its Hunter Valley line. The trains have since been phased out in favour of the newer Endeavour railcar model. The 660/760 railcars were converted by NSWGR from earlier 600/700 Class vehicles between 1973 to 1975.
The 600 Class railcars, built in 1949-1950, were not compatible electrically with the later 900/950 and 620 Class diesel trains. In 1972, NSWGR 22 purchased Cummins diesel engines and Twin Disc transmissions with the intention of re-engining the entire ten units of the class and making them compatible with their later sisters. Continued mechanical failures of the early 620 Class units resulted in some of these engines and transmissions being diverted to upgrading the 620 Class and in the end only five of the ten 600 Class sets were converted.
The converted units were fitted with two 300 hp Cummins NT-855-R2 in-line six cylinder, four stroke diesel engines coupled to Twin Disc automatic lock-up torque converters. They were also fitted with 620 Class control systems and were then able to work in multiple with the 900/950 and 620 Class units. During the reconstruction, side mounted radiators were fitted in the centre of the power car, however, the original window arrangement was not changed. The louvred side radiator vents are an aid to distinguish them from their earlier 600 Class sisters. Apart from control system and driver's cab changes, the layout of the trailer cars was unaltered.
The units were renumbered as the 660/760 Class by the simple expedient of adding 60 to their original numbers. The units converted were 601/701, 605/705, 608/708, 609/709 and 610/710. 601 becoming 661, 605 becoming 665, etc.
Once converted, the 660/760's worked interchangeably with the 620 Class on Newcastle, Wollongong and outer Sydney suburban services. Like the 620 Class, the 660's were re-engined with more powerful 335 hp Cummins diesel engines coupled to Voith automatic lock-up torque converters in the 1980s.
All sets were withdrawn by 1994 and while some have been preserved in heritage rolling stock collections, none are now operational.