(CNN) -- [Updated 4 p.m. Monday, February 6] This overcrowded train is in Jakarta, Indonesia, where it is common to see people riding on the roofs of cars in the state-run rail system.
To discourage "train surfing," the railway has begun to install concrete balls over some tracks. The balls are connected to chains high above the track but just over the height of the train itself. You can check out the setup here.
"Roof surfers will hit the balls, and the balls are enough to crack someone's head," Eman Sulaiman, chief of the Bekasi city station, told the Jakarta Globe last month.
Many commuters jump on the roof because the train cars are full. Some do it to avoid paying a fare. Others do it for the thrill.
The railway said it resorted to the balls because other strategies, such as oiling the roofs, didn't work. Critics believe this latest effort is too harsh.
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CNN's Brad Lendon contributed to this report.