It flashes metallically out of the darkness, a row of silvery shimmering points shows us the way into the night.

It's the reflections of the LED headlights at the contact points between the gears and the rack, scoured clean from the daily use of the "Zage".

That's what Stuttgart residents and experts call the only rack railway in Germany that is part of an urban public transport network.

It is considered a Stuttgart landmark, is popular with tourists because of the spectacular route with panoramic views and still plays an important role in inner-city local passenger transport.

Since 1978, the 2.2-kilometre-long, meter-gauge railway has been run by the Stuttgart trams (SSB) as line 10 in the timetable.

The start is down in the valley at Marienplatz, the destination station is called Degerloch.

Even a technical legend has to be updated from time to time when it carries 2,500 passengers a day, including commuter and school traffic.

For this reason, the single-track line with its only, elaborately constructed siding at the Wielandshöhe has been bustling with activity for many nights this year.

Actually, the operation of the Zacke ends around 9 p.m., the shutdown then lasts until early the next morning.

In the meantime, 10 minibuses are running on the route.

But in the summer of 2022, Stuttgart will also show its teeth after business hours.

While the old ZT 4.1 type cars (four-axle rack-and-pinion multiple units, first generation) from 1981 are in regular service during the day, the SSB, together with the Swiss manufacturer Stadler Rail from Bussnang, carry out approval runs with the new ZT 4.2 rack-type multiple-unit cars after the end of operations.

According to Stadler, it is the world market leader for rack railways.

The company's engineers then unfold their laptops and connect them to the control system, while the seats are occupied by sand-filled canisters to simulate the mass of passengers.

Various test scenarios explore how the new vehicles behave in everyday life and in critical situations.

Individual from the ground up

There is a tradition of looking to Switzerland for new acquisitions in the state capital of Baden-Württemberg.

The ZT 4.1 that went into operation more than 40 years ago had a car body from MAN Nuremberg and the electrics came from AEG.

At that time, however, the gear drive was supplied by the Schweizerische Lokomotiv- und Maschinenfabrik Winterthur (SLM) - and their gear wheel division has belonged to Stadler since 1998, where the complete railcars are now being built.

At Stadler, rack railways are among the "tailor-made" products, i.e. tailor-made rail vehicles.

In fact, they are considered to be so technically individual that they cannot be bought off the shelf.

Other railway vehicles are often slightly adapted for the specific purpose.

But here the degree of customization is particularly high.

Only special forms such as suspension railways, the best-known German example of which operates in Wuppertal, are likely to be even more unusual.

This is also the reason why the new Zacke vehicle generation is being approved directly by the customer, explains SSB project manager Detlev Martin.