[srl]

The are, with the effect (and I'm guessing intent) that an electrical/motor failure won't lead to the elevator plunging 20 stories and killing all occupants.

Software engineering could (and should) learn a few things from mechanical engineering. People have been saying that for decades, but it bears repeating...

[nialo]

There's a separate emergency brake system to prevent the elevator from falling, because you also need to be safe if the cables holding the elevator up break (this is actually the technology that lead to the first elevators). The counterweight is probably primarily just to reduce power/energy requirements.

[ww520]

Crash driven design. Design your software such that it can recover from a crash to a known state and continue. Make life so much simpler. Shutdown is just a matter of power off. Startup always does a recovery before starting.