Why Can't I Bring Metallic Balloons on the MTR?

We’ve all tittered over the signs and announcements banning metallic balloons from the MTR. After all, it feels like another of those spurious instructions that Hong Kong is so happy to throw at us, another example of rules for the sake of rules.

But there’s actually a reason why metallic balloons aren’t a good idea on our underground. You see, it all goes back to the way that the MTR is powered. Most subway systems around the world use a third-rail system, in which power is transmitted along an additional rail on the tracks.

It’s fine, but it’s also limited: You can get a comparatively lower voltage on it, which means slower speeds for your train—and less powerful air-conditioning.

Also, there’s a giant metal strip filled with electricity running along the ground, which means if anyone falls or wanders onto the track and touches it, they’ve got a good chance of fatally electrocuting themself.

The MTR runs its cables overhead, just as the trams do. That means it can carry higher voltages, and it’s also out of the way of all but the most enterprising of suicidal trespassers. But those overhead power cables have their downside—and here’s where the metallic balloons come in.

Metallic balloons are almost always filled with helium, allowing them to float. But of course, hand a child a floaty balloon and one thing is guaranteed—they’ll let go. Metallic balloons run the risk of drifting into the tunnels, where their conductive surfaces are likely to short-circuit the power lines.