Plans for north-south railways across central London go back to the 1940s at least, when there were several proposals in the 1943 County of London Plan which were developed further in a following report in 1946.

The Victoria line, which opened in stages from 1968, was one of the routes suggested in these plans, but reviving the Farringdon to Blackfriars route for passenger trains began to be considered seriously in the 1970s.
The British Railways Board then developed plans for what would become Thameslink, and the newly-created business sector of Network SouthEast inherited responsibility for the project in 1986.
Services between Bedford, Farringdon, Blackfriars and Brighton began under the Thameslink brand in 1988.

As the Thameslink service was to use a route with 25 kV AC OHLE north of Farringdon and along the branch to Moorgate, and 750 V DC third-rail electrification south of Farringdon, the Class 319 trains were equipped for dual-voltage operation, making them very versatile.
They were also the first British Rail units to use modern thyristor control in place of a camshaft and resistor bank.

The body shape of the Class 319 is slightly different from contemporary electric units due to restrictions in the loading gauge in Kings Cross Tunnel, which meant that other dual-voltage units were not suitable.
 They were also required to have emergency end doors in the cabs, due to the twin single-bore layout of Smithfield tunnel preventing normal train evacuation.

Two sub-classes of Class 319 units, 60 Class 319/0s and 26 Class 319/1s, were originally built.
Over the years, units have been refurbished, creating five sub-classes, of which four still exist.

Class 321 passenger units and Class 325 postal units were developed from the Class 319 design, using similar traction equipment and the same steel body design, with revised cab designs, with the 325 using a Networker style cab design.
Plans for north-south railways across central London go back to the 1940s at least, when there were several proposals in the 1943 County of London Plan which were developed further in a following report in 1946.
The Victoria line, which opened in stages from 1968, was one of the routes suggested in these plans, but reviving the Farringdon to Blackfriars route for passenger trains began to be considered seriously in the 1970s. The British Railways Board then developed plans for what would become Thameslink, and the newly-created business sector of Network SouthEast inherited responsibility for the project in 1986. Services between Bedford, Farringdon, Blackfriars and Brighton began under the Thameslink brand in 1988.
As the Thameslink service was to use a route with 25 kV AC OHLE north of Farringdon and along the branch to Moorgate, and 750 V DC third-rail electrification south of Farringdon, the Class 319 trains were equipped for dual-voltage operation, making them very versatile. They were also the first British Rail units to use modern thyristor control in place of a camshaft and resistor bank.
The body shape of the Class 319 is slightly different from contemporary electric units due to restrictions in the loading gauge in Kings Cross Tunnel, which meant that other dual-voltage units were not suitable.  They were also required to have emergency end doors in the cabs, due to the twin single-bore layout of Smithfield tunnel preventing normal train evacuation.
Two sub-classes of Class 319 units, 60 Class 319/0s and 26 Class 319/1s, were originally built. Over the years, units have been refurbished, creating five sub-classes, of which four still exist.
Class 321 passenger units and Class 325 postal units were developed from the Class 319 design, using similar traction equipment and the same steel body design, with revised cab designs, with the 325 using a Networker style cab design.
British Rail Class 319