Obviously careful consideration on the design, implementation, and security of the system.
He configured a TV remote control so that it would get trams to change tracks instead of flipping channels on the telly. The result of his fun was several accidents and four derailments, but fortunately no fatalities.
When you look at how transportation security is disproportionately allocated against specific terrorist threats, it makes you wonder how long it takes before someone attempts a malicious attack along these lines.
[photo by: Vitelot]