ProRail has cut the number of high speed intercity trains between Amsterdam and Rotterdam to two an hour instead of the usual four because of track safety concerns.

A quick scan of the viaducts and bridges along the track revealed that five of them are showing signs of deterioration, such as cracks, and it would be dangerous to continue to expose them to high speeds, ProRail said. The new schedule started on Thursday.

Intercity trains will have to reduce their speed to 120 kph, and in one place to 80 kph and international services will be slower, leading to timetable cuts, ProRail said. There will be no changes to the number of intercity trains to Brussels but they will take longer.

ProRail is investigating the seriousness of the cracks but said it is still unclear how long it will take to solve the problems.

Train operator NS said the news comes as a “big disappointment” for travelers. “We had hoped we could run the trains at full speed sooner rather than later,” a spokesman told news platform Nu.nl.

“The problems plaguing this line have gone on for too long,” NS chief Wouter Koolmees commented. “We see that ProRail is taking this very seriously and of course safety is paramount, but we do hope they will remedy the situation as quickly as possible,” he told the news site.

Rail user lobby Rover said it was ‘unpleasantly surprised” by the news and said that if the problem persists, NS should recompense travelers, for instance by dropping the current surcharge on the service.