targeting not only first-generation DMUs but a number of locomotive-hauled trains as well. It had also been observed that, in its current configuration, the Class 150 would be unsatisfactory in some criteria for more-upmarket services, but that some thought into developing derivatives of the type to handle such services had been made. One early solution for reducing internal noise levels was the discarding of openable windows in favour of fully-sealed units, along with the relocation of the external doors into vestibules located at either ends of each coach. Furthermore, the coaches could be stretched, providing more internal volume and thus enabling the somewhat cramped two-by-three seating arrangement of the Class 150 to be substituted with a more roomy two-by-two counterpart. These changes could be implemented without impacting much of the benefits of adopting the existing design. It was identified that this would result in a weight increase and thus a decreased power-to-weight ratio. Studies determined that the performance of the proposed DMU showed only minor change, and would achieve similar journey times across the intended cross-country routes to the Class 150. It was also found that, while there was a slight increase in fuel consumption due to the modifications, the envisioned DMU possessed significantly less fuel consumption than locomotive-hauled trains as well as reduced maintenance costs. Accordingly, it was decided to proceed with developing a detailed specification and issuing it to industry. The Class 158 is a two- or three-car diesel unit designed for regional express services. The bodyshells are aluminium with doors at each end of the passenger saloon. Each vehicle is fitted with a Cummins or Perkins turbo-diesel engine that drives both axles on the inner bogie via a Voith T211 two-speed hydrokinetic transmission and Gmeinder GM190 final drive unit. The engines were rated at either or , depending on the model. Maximum speed is . Most units were built with two coaches, but a batch of units contained an additional centre car for the busy Transpennine Route. Each vehicle is fitted with a BSI autocoupler at both ends; however, only the cab ends have automatic electrical connecters. This allowed three-car sets to be formed by inserting an additional driving car into a set with an adaptor for two different gangway sizes. Description Variants A total of 182 Class 158 units were constructed. The majority were built as two-car sets. 17 units were produced as three-car units; eight