This paper shows that the particular conditions in which a cellular phone network is operated underground can make it capture a significant part of particular events like entries in tunnels. With the help of the mobile network operator Orange, some experiments have been conducted in Paris' underground transit system. Results show that good estimates of dynamic quantities, such as travel time, passenger flow and train occupancy, can be derived from cellular data. Those estimates are used to compute two quality of service indicators: the interstation passenger density per train, and the passenger delay between an origin and a destination.