No doubt they are possible. One factor that discourages their existence though is that the dregs of society mess them up or go in there to do drugs in private and then conk out. It's the number one pushback to providing restrooms for the public. It sucks, but it's how it is. Most likely the only way around this is to stop subsidizing transit and to be very careful to enforce fares. This is not popular among the people who love transit, who tend to also love the underdogs and root for them.