drum118
Superstar
Its not the only system to do this. It also done by time as well.London is probably the best example of a mandatory tap-out. In fact, the gates won't even open if you don't have enough money on the card to pay for that very fare.
We ran into a few gates where our card fail to be read and able to get out after the card was check by attendant at a special gate area.
We also had a case in Paris trying to get in to the station and no one around to deal with the issue. We had no clue where we could go to get into this station as we needed it to get us to the train station to catch our train for the next city to the point, we slide under the gate to get in since very few people where using this entrance.