No balloons in metro system. This time a star shaped one!
This is another version of the Hong Kong one (I’m still missing a sad kid with the balloon)
After searching a little online, it looks like a lost metallic balloon can be a real problem for an underground system, as it happened in 1996 in Hong Kong. The lost balloon can get up and short-cut the power lines of the trains, leaving the hole system stopped for a while. It’s really well explained here.
Seen in Metro de Madrid, Spain
Thanks Mr. Leandro for the pic!