There's a difference, though: democracies (in theory) are designed to allow the majority to exert power, while unions are there to allow minorities to exert power. In this case, assuming the parent's assertion is true, you have a union that is exerting power for its own benefit, at a huge detriment to the majority. We can collectively decide that this is ok, if we want: if we value the continued employment of union members over a more efficient subway system, then that's fine. If not, then we're in trouble.

Hell, I'd be fine paying the unionized subway employees for the rest of their lives to sit at home doing literally nothing if that was the only way to get them to agree to automating their jobs away. At least then we'd have the outcome we want, and not have to pay the next several generations of subway operators to do a job that holds the system back.