The British Rail Class 303 electric multiple units, also known as "Blue Train" units, were introduced in 1960 for the electrification of the North Clyde and the Cathcart Circle lines in Strathclyde. They were initially classified as AM3 units before the introduction of the TOPS classification system, and were the dominant EMU on the Glasgow suburban railway network for over 25 years before being progressively phased out by newer rolling stock. The final units were withdrawn from service in 2002. The fleet's lifespan was 42 years.

The units were later used on the Inverclyde and Argyle lines of the Glasgow suburban railway network as various electrification schemes came to fruition.

One complete unit has been saved for preservation. It is a hybrid unit consisting of the driving vehicles from set 303032 and the motor coach from 303023, which replaced 303032's own damaged motor coach. Unit 303023 was one of only four to carry the later SPT carmine/cream livery. The set has been modified to operate in multiple with a blue-star compatible diesel locomotive (typically a Class 27) so that it can be driven on an unelectrified heritage line.