The Class 320 is effectively a 3-car derivative of the Class 321 units found in and around London and the Midlands. Built in 1990 by BREL in York, 22 3-car sets were ordered by SPT to supersede the aging Class 303 and Class 311 stock which were by then 30 years old. The Class 320 units are fitted with GSM-R cab radios and took part in the GSM-R trial in the Strathclyde area.