On the other hand, there are several tropical species of this genus which are commonly sold as house plants, and over the years many of these have probably been brought into the subway system by shoppers. Perhaps spores from one of these got things going.
Whatever they are, it's a surprise to see them deep in the subway system. They are abundant and easily spotted at several stops. Are they confined to the Red Line? I don't know; perhaps someone reading this will comment.