Together with the Rhine-Main S-Bahn and the tram network, it forms the backbone of the public transport system in Frankfurt.

On 5 April 1960, the SPD faction requested that the city council might commit to a two-line straddle-beam monorail system designed by Alweg.

Lord Mayor Werner Bockelmann, however, advocated from the outset the construction of a U-Bahn, which was however considered the most expensive option.

The decision was finally made in late 1961 in favour of a U-Bahn system, which was to be built in several sections initially using existing tram infrastructure.

In the first construction phase, the tunnels of the inner city were to be built for the time being, which were to be connected via provisional ramps to the adjacent tram routes.

It was not until the second construction phase that the tunnels were to be extended beyond the inner city and connected to suitable upgraded above-ground routes in the suburbs.

For the first stage of development until 1975 - approximately corresponding to today's main lines A and B - construction costs of 565 million DM were expected.

The new Lord Mayor Willi Brundert compared the "boldly begun subway construction" in 1964 in its dimensions with the medieval cathedral building.

Most of the other above-ground sections run on reserved track but have numerous level crossings for road traffic and pedestrians.

As of 2024[update] extensions of the system in planning or under construction include: There are two depots for U-Bahn trains: The U1 Class consists of two six-axle, two-section prototype vehicles built by Duewag in 1965,[9] derived from the manufacturer's previous tramcars.

[9] The final (seventh) batch of seven units was delivered after a fire at Heddernheim Depot in 1980 destroyed five sets.

[7] They were originally painted in a red and white livery, before being repainted in beige, ivory and grey from 1981 and finally from 1996 turquoise (officially known as subaru vista blue).

This had slightly higher platforms (87 cm) and these vehicles were rebuilt as U2e with the steps in the door areas removed.

[7] Siemens adapted the U2 design for the North American light rail market and similar vehicles were supplied to Edmonton, Calgary and San Diego.

The U3 Class was based on the U2, but slightly longer (24.49 m), with a body made entirely of steel, and without any steps for access from low platforms.

An accident involving the two cars 517 and 532 on 28 February 2007 resulted in their early retirement from the fleet and being scrapped for spare parts.

They were fitted with new yellow handrails and orange entrance areas to help visually impaired passengers orientate themselves in the train, and an air conditioning system for the driver's cab.

[20] The U5 Class, the newest of the U-Bahn fleet, has been produced by Bombardier Transportation (now Alstom) in Bautzen, and is part of the manufacturer's Flexity Swift range.

[27] To differentiate the two subclasses, the batch with folding steps was designated Pt (t for tunnelgängig, operable in tunnels).

Even after no other lines required the folding steps, the Ptb remained indispensable on the U5 service where on certain sections the stations did not have high platforms until 2016.