The British Rail Class 142 diesel multiple unit passenger railbuses were built for British Rail (BR) from 1985 to 1987. The class were built with a high level of commonality with the widely-used Leyland National bus. They are part of the Pacer family of railbuses. The last set was withdrawn from service in 2020.

The Class 142 shared a high degree of similarity to the design of the Class 141. However, one major area of change is that both the Class 142 and Class 143 featured a noticeably wider body, instead of adhering to the width of the standard bus as per the Class 141; specifically, the width was expanded to the maximum amount permissible to remain within the loading gauge. This resulted in an increased internal area to accommodate passengers within, enabling a three-by-two seating arrangement to be installed for a total maximum capacity of 121 seats per set. The increased seating was particularly useful as, in addition to their use on rural feeder services, the Class 142's use on short range urban services had been foreseen by BR planners.

Several operational class 142 units have made it into preservation and additional sets are set to be saved/earmarked for preservation, most of which so far being ex-Northern operated units with one being an ex-Arriva Trains Wales unit. The pioneer Class 142 Unit, 142001, is part of the National Collection and preserved at the National Railway Museum Shildon. Other select units have also been purchased but will only be used to provide spare parts to the units which are planned to operate on heritage railways or museums.