the class 123 was a design of diesel multiple unit built for british rail in 1963. they were built by british rail at swindon works, and designed like all swindon units as inter-city sets. ten of the four-car sets were built and introduced in 1963. the units bear a visible similarity to the british rail class 309, however there is no 'relation' here as the two types were built by different manufacturers for different markets.