There's a ton cities can do to make public transit a lot more comfortable without billions for building out new public transit infrastructure (which isn't to say that the latter is a bad idea!). Like, put some police on trains and buses so people are less likely to stab, urinate, smoke, blast music, etc. Clean buses and train cars regularly. Keep a high standard for antisocial behavior (if someone is being disruptive, kick them off). Run more trains and buses on existing infrastructure (don't make people wait half an hour for a train, keep people from being packed so tightly into train cars and buses). Keep reliability high.

If we can't manage these fundamentals, I don't know how much of an improvement we would see by building out new infrastructure.