[citation needed] The route continues through the Altstadt, first under Weißfrauenstraße, then Münzgasse and Limpurgergasse to reach the Römerberg, the central square of the old town.The station, called Dom / Römer, lies deep below the oldest settlement in Frankfurt.

Beyond Dom / Römer station, the route runs under the cathedral square and Kannengießergasse to Fahrgasse, which it crosses before swinging in a wide arc under two blocks and the Dominican monastery in a northerly direction to follow the course of the Kurt-Schumacher-Straße and reach Konstablerwache.

Unlike this, today's route no longer runs directly to the center of Preungesheim, but to a residential area characterised by its 1950s buildings.

Towards the west, the single-track section of the former tram services 13 and 22 leads over the Marbachweg to the Eschersheimer Landstraße, where it connects at the Dornbusch station to the A-line, which is also at street level.

The station at Bornheim Mitte, located where Berger Straße crosses Saalburgstraße, is the most important public transport node in the district and provides connections to trams in Frankfurt am Main and buses.

Beyond Seckbacher Landstraße the tracks leave the tunnel and continue above ground to Depot East (German: Betriebshof Ost) and a connection to the C Line.

It was the first U-Bahn service in Frankfurt to run entirely underground, linking the central railway station to Seckbacher Landstraße in Bornheim.

After six months of trial operation the line was permanently extended to Enkheim in December 2008,[5] sharing the existing ground-level tracks used by the U7 service.

It began operation, as the B1, on 26 May 1974 between Willy-Brandt-Platz (then called Theaterplatz) and Gießener Straße, and was extended to Preungesheim in 1977 and the central railway station in 1978 when these sections of the B Line opened.

After a ceremony and a laying of the foundation stone in the underground station Miquel- / Adickesallee, where 1963 the first Rammschlag took place, the participants, including Lord Mayor Brundert, Möller Department of Transport and Minister President Zinn went to Friedberger Tor, where, accompanied by speeches, the Straßenbahnerkapelle and Freibier the ceremonial start of construction of the B stretch took place.

The line was the first tunnel built in mining style in Frankfurt, this design was later due to the significantly lower loads on the surface to become the rule.

On the east side of the Römerberg, at the site of the later reconstructed half-timbered line, two buildings were built, which were demolished again with the beginning of the subway construction.

In 1963, an urban development competition was held for the construction of the Dom-Römer area, which was decided in favor of a large, modern building complex.

The subject of the competition was also the design of the entrances to the planned subway station Römer underneath the grounds as well as the underground car park to be built next to it.

The excavation pit was located in the historic heart of the city; for thousands of years man-inhabited settlement soil was destroyed for archaeological research.

For almost ten years, until the reconstruction of the Römerberg-Ostzeile and the construction of the Kunsthalle Schirn, the then so-called Höckerzone occupied the historic urban space between the Kaiserdom and Römerberg.

The eastern exit of the station leads over a very long escalator to the outside, the passenger rolls while the whole time directly to the tower of the cathedral, which is a rather unusual way of approaching a medieval building.

From the Theaterplatz to the Konstablerwache, the tunnel in the ramparts led to the surface, from where the train continued as a normal tram in the car road of Eckenheimer Landstrasse.

These were 30 cm narrower compared to the type U2 used on the A-line and at that time had novel swivel stages on the doors, so that they could also be used on tram routes without adapting the tracks.

Under the carriageway of the station forecourt, over which numerous tram routes and the federal highway 44 lead, was initially created here again a very large underground shopping arcade (B-level).

These facilities were again built in an open pit, for many years was instead of Bahnhofsplatzes a deep hole, the traffic was diverted, the northern part of the historic station building, similar to the previously baroque main guard, removed, stored and rebuilt after completion of construction.

As resistance to the planned construction of barrier-free elevated platforms increased along the above-ground section in the Northrend, an opinion was commissioned in 2007 to investigate whether the route from Preungesheim to Konstablerwache was integrated into the tram network at the initiative of the black-green city government could be.

Nevertheless, the tunneling station would nevertheless have been provided as an end point, as a result of which the line shortening of 1980 along with the associated operational problems (single-lane terminal) would have been repeated in practice.

In 2008, the report was extended to include a plan case for a second line from Preungesheim via Glauburgstraße, Friedberger Landstraße, Konstablerwache (above ground) and the old town section to the central railway station.

The new proposal of the city, neither mid-high platforms nor Troglösungen to build, but instead at the two stops Musterschule and Glauburgstraße two staggered, each 72 meters long and up to 80 cm high Seitenbahnsteige to meet, met with varying degrees of resonance.

While the supporters of an underground railway operating in the city center greeted the concept, resistance was provoked in the affected district of Nordend.

In the Frankfurt city council meeting of July 1, 2010, the proposal, which was once again optimized in the spring of 2010, found broad support in the parliamentary groups of the CDU, SPD, FDP, Greens, and Republicans against leftists, the FAG, the Free Voters, and the NPD.

The rebuilding of the station Preungesheim took place from July to November 2015 and takes up the pre-planning for the extension to the Frankfurt mountain by the positions of side and middle platform were exchanged compared to the original condition.

The 1.7 km (1.1 mi) long route will run along the Homburger Landstraße and have intermediate stations at August-Schanz-Straße and Berkersheimer Weg.

Due to the low projected cost-benefit factor and the resulting poor prospects for federal and state funding, implementation of this extension is unlikely.