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

Ninety-one three-car units were built by Pressed Steel at Linwood near Paisley, from 1959 to 1961, and they were introduced into service in 1960.

There was no electrified connection between the two networks until late in the life of the trains, and thus transfers between the two halves of the system, performed quite regularly, were dragged by locomotives via the Shields Road-High Street line.

Based on the Mark 1 bodyshell design, the Class 303 units utilised electrical gear made by Metropolitan-Vickers (Metrovick).

[9] Over the weekend of 17/18 December 1960, all 72 EMU sets were taken into storage and the old steam-operated service was temporarily reinstated whilst urgent modifications were made.

When built, the driving cabs had distinctive wrap-around front windows, although these were replaced by flat, toughened glass in the 1970s to better protect drivers following some incidents of stone-throwing vandalism.

Glass partitions behind the cabs allowed passengers in the front and rearmost carriages to see the drivers' view of the track.

The glass bulkheads behind the driving cabs were another casualty of the refurbishment - passengers could no longer see the driver's view through the front windows.

Following refurbishment, units were repainted in the striking new orange and black livery introduced by the newly created Strathclyde PTE.

Initially, they were used on the Crewe to Liverpool service but were soon transferred to the Manchester area, operating services from Manchester Piccadilly to Altrincham, Hazel Grove, Macclesfield, Alderley Edge, Crewe and on the line to Glossop and Hadfield - this line had recently been converted from 1,500 V DC.

By now electrification around Glasgow had spread, and units could be found working on many routes, such as those to Gourock, Motherwell, Coatbridge and Ardrossan Town.

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.