Different modes of fare collection can be found in the world's metro systems. Physically, there may be either fare gates or the so-called honour system, with modes of payment reaching from token to smartcard.
Fare gates or honour system.
Gates separate the paid area physically. The majority of the world's metros uses this system. Smartcard payment is straightforward with gates as people are driven to touch their cards when entering and when leaving the paid area.
Disadvantages for the handicapped, the pregnant, and people with baggage. Broken turnstiles can cause rush-hour bottlenecks. In case of fire or panic, gates must open. Misuse of emergency opening needs to be prevented, usually through the presence of staff.
Also called proof-of-payment (POP) system. The paid area is marked by signs or stripes on the floor. Ticket inspectors randomly check tickets on-train, sometimes working undercover (without uniforms). Passengers without a ticket will be fined. The loss in revenue by fare evasers might be absorbed by the income through these fines.
Advantages: Stations need not be staffed.
Disadvantages: More fare dodging. People can just forget to pay and get caught. Ticket inspectors, though usually operating in teams, are in danger to get beaten. Smart Card systems are not straightforward with the honour system as passengers can easily forget to swipe their cards at exits.
Smartcard, RFID token, token, free. The classical mode of payment, the paper or magnet strip ticket, is almost ubiquitous, in addition to other payment systems.
This is a debit card which operates a reader at each fare gate approached. The reader will open the gate and record the start point of the journey and then record the exit point and deduct the required fare.
Advantage: Can automatically grant discounts, e.g. stop increasing the fare when the amount of a day pass has been reached.
Disadvantage: Requires fare gates to work smoothly. Can be misused to track passenger's journeys.
Similar to Smart Card. A coin-sized plastic chip contains a so-called RFID tag. This enables the system to calculate the amount paid by the passenger and the fare. The chip coin has to be touched with the entry gate and inserted in the exit gate. When the fare for the journey is correct, the token is retained by the exit gate and the gate opens. When the fare is insufficient, the exit gate will reject the coin and the passenger has to recharge it at a machine.
Advantages: Combines the portability and robustness of a token with the advanced features of a smart card.