No, not a joke, although it sure sounds like the punchline for one. Apparently the problem of people riding the tops of trains in Indonesia has become so bad that railway officials are testing hanging concrete balls overhead to deter train riders.
The article says that so far officials have not seen anyone try to ride the train on lines protected by the concrete balls. The officials say they are trying to prevent the frequent deaths that occur when illegal train-riders fall off or between cars. Indonesia's trains are frequently overcrowded.
There are numerous news articles about this online. Here's one:
Note in the video that the riders are on top of electrically-powered trains. I can't see how concrete balls are going to work in this case, because of course they would tangle up in the pantographs.
Railfans will notice that these danging balls bear a remarkable similarity to tell-tales. What's that, you ask? Well, in the good old days before automatic air brakes, brakemen would run across the tops of cars to manually crank brakes on or off individual freight cars. Now, imagine a brakeman on top of the car, unaware of a tunnel around the corner... a tell-tale was a set of ropes dangling from a structure placed ahead of any overhead obstruction. The ropes would smack the brakeman in the back and give enough warning for him to lay down to avoid the obstruction.
The amusingly-named web site irritatedvowel.com has this photo of a Western Maryland tell-tale, apparently a net-like device.
Given that many photos I see of train-riders in Indonesia show them hanging from the sides, front and back of the trains too, this may not help a lot.
There's no easy answer... except more trains and less congestion.
PS - do you suppose I will show up in Google searches for "concrete balls" now?