In the month since the city’s rapid transit system has been running at reduced capacity, police have issued 400 graffiti-related citations, the paper reported. While an increase in graffiti artists are usually expected during school holidays, the deserted train tracks of the S-Bahn have exacerbated the problem for police and rail officials.

In response to the colourful problem, S-Bahn operator Deutsche Bahn (DB) and police have increased security around the empty platforms and tracks throughout the city. Lines servicing more suburban parts of the city are supervised with helicopters at night while track beds are watched by thermal-imaging cameras.

But with all the extra attention at night, artists are finding the light of day the best time to leave their mark, the paper said. On Wednesday, two children in the community of Zehlendorf were caught by police with spray paint in hand at 6:20 pm, having already used two entire train cars as their canvas.

Another man was also caught at the Westkreuz station tagging a train at 5 pm on Thursday, the paper said.

Officials are also warning of the dangers that come with exploring the rails. Many lines are still electrified and create a shock risk.

Police expect a flood of new tags next week when a Friedrichshain-district hip-hop festival will attract more artists with a graffiti contest. Every year, 15,000 new graffiti paintings appear on Berlin’s surfaces, making it a destination for street artists worldwide.

The capital city’s rapid transit system has been out of service in Berlin’s main east to west access since July 20. DB, the national rail service provider, was ordered to conduct wheel repairs on 930 of the train cars by the national rail watchdog (EBA).

Most of the routes are to come back in service at reduced capacity by Monday, August 3, with most train lines running every 20 minutes. DB brought train cars in from Stuttgart and Munich to service some of the suburban lines, but because passengers were unaware of the trains running there, most remained empty.