Teens are relevant here mostly as a test of the system’s ability to exclude. A system excluding people sounds a little mean, but it is important if you want to have a transit system that is actually fit for transit.

When there’s a homeless man shooting up with some drugs on the narrow platform at Bleecker St while a crowd of people try to get off the train and get out of there without tripping or provoking a confrontation, and that’s “normal” for the city subway, the people may decide the system is no longer fit for transit. Fortunately that’s still “slightly extraordinary” for NYC and not quite “normal” (I only saw him once) but it’s all very unfortunate.

Replace 2 teenagers with "homeless guy that shit himself" or "criminals" [1] and you have a different story. If you have proper security this sort of class of behavior is entirely prevented.

[1]: https://www.sfgate.com/crime/article/BART-takeover-robbery-5...

Just a thought, but maybe a social safety net would be more effective in preventing the homeless from getting into the kind of state where they do that? Rather than more security. Just thinking laterally about the problem here..

We can do both at the same time. We can help the homeless and also prevent people from shitting or littering in public transportation vehicles and on boarding platforms.