Karlsruhe ran trams onto mainline rail tracks, and ridership on the Bretten line rose 400%

Works in Progress described Karlsruhe, a German city of about 300,000, whose main station lies nearly two kilometres from the old centre after an early twentieth-century rebuild, so passengers had to change to the tram. In 1957 the city took over a bankrupt suburban line, the Albtalbahn, and extended it into the centre over the tram network. Over four years the gauge was converted from 1,000 mm to 1,435 mm and the line was re-electrified at tram voltage. The 70-minute journey to Bad Herrenalb dropped to 46 minutes and passengers no longer had to change.

The inventive step came later: tram-trains were run onto railway lines that carry other traffic. In 1979 the line was extended onto a freight railway north of the city, and in 1992 the first tram-train took over existing passenger services to Bretten. Joining the networks was hard. German railways use 15 kV AC while the trams use 750 V DC, so dual-voltage vehicles, rare then and now, were needed. The two networks also had different signalling, and retractable steps bridge the gap between 2.65 m-wide trams and platforms built for 3 m-wide trains and different platform heights.

The article notes trade-offs: trams share road space, so Karlsruhe runs them on wide suburban roads with their own lanes and gives them signal priority. Tram-trains reach speeds about the same as the Paris Metro, but the 75 m vehicles carry about 8,800 passengers per hour at peak on the busiest section, far less than Munich's S-Bahn.

Join the railway to the tram network with dual-voltage tram-trains, so suburban lines run through the centre without a tunnel.

It reuses suburban railway lines that already exist, so only a link to the tram network is needed.

Dual-voltage vehicles remove the need to change at the edge of the centre.

Running through the centre avoids the space a terminating train needs to turn around.

Own lanes on wide suburban streets and signal priority at junctions offset the speed cost of sharing roads.

Linking two existing networks with vehicles that work on both can beat building a new one, when each network already covers half the journey.

Per Works in Progress, Bretten line ridership rose 400% after the extension, to 18,000 trips a day from 2,000. Passenger numbers on the network doubled between 1985 and 1999 as lines were added. The network now carries 170 million passengers a year over more than 660 km. As ridership grew, a 2.1 km tram-train tunnel under the centre was completed in 2021–2022, allowing the main shopping street to be pedestrianised.