The British Rail Class 423 ( or 4Vep ) electrical multiple units were built by British Rail ( BR ) at York Works from 1967 to 1974 , although the MBSOs and TSOs of the first 20 , 7701-7720 , were constructed at Derby Works .
The British Rail Class 423 ( or 4Vep ) electrical multiple units were built by BR at York Works from 1967 to 1974 , although the MBSOs and TSOs of the first 20 , 7701-7720 , were constructed at Derby Works .