This fleet of 91 3-car sets were built by Pressed steel in 1959-61 for the electrification of the Glasgow North and South side routes. The original order was for 35 sets, which was followed by a subsequent order for 56 sets.
The sets emerged painted in a new livery for BR passenger stock, that of Caledonian Blue. Over the years this gave way to standard BR rail blue.
A major refurbishment scheme took place in the late 1980s which saw sets emerged in Strathclyde orange livery, the refurbishing work saw gangways fitted internally and improvements to the passenger environment. In the mid 1990s the latest Strathclyde red and cream livery was applied to a handful of sets.
Replacement of the sets following introduction of new Class 334s sets to the Ayrshire line in 2001-2002 has seen cascading of other classes allowing the 303s to be phased out of traffic.
From the late 1970s several Class 303s sets were reallocated to the London Midland Region for operation in the Crewe-Liverpool area.

The original allocation of these sets was 001-056 - North Side and 057-091 - South Side, these largely clanged from 1967.

The Scottish Region issued sub sub classifications in 1971-72, these were:
303/1 - standard units
303/2 - set 035 fitted with Deans doors
303/3 - set 071 fitted with silicon rectifiers and oil cooled fans
303/4 - set 035 (again) thyristor control, oil cooled rectifiers (Deans doors removed in 1971)
303/5 - sets 072-080 fitted with silicon rectifiers with oil cooling of transformer and rectifier
303/6 - sets 040/044/046-070/081-091 fitted with silicon rectifiers with separate oil cooling for each
303/7 - sets 001-034/046-039/041-043/045 fitted with air cooled silicon rectifiers