Prohibition sign in Schiphol airport train station, The Netherlands. 
Loose balloons can strike train power lines, causing delays across  the railway network.
<a href="//commons.wikimedia.org/wiki/User:Wittylama" title="User:Wittylama">Wittylama</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>