i had the opportunity to experience the Paris subway system this past week (strictly there on business - i take no pleasure in such a country...well i did have a pretty good time and at the government's expense) and noticed at least 3 totally different types of trains running in the system - a small train, a large (regional) train, and then this train that ran on rubber tires - like the one in mexico city (which by the way is quite an elegant subway system of its own) - seems like a logistical/managerial/maintenance nightmare.