A feature is the use of rubber-tired trains on five lines: this technique was developed by RATP and entered service in 1951. The technology was exported to many networks around the world (including Montreal, Mexico City and Santiago). Lines 1, 4, 6, 11 and 14 have special adaptations to accommodate rubber-tyred trains. Trains are composed of 3 to 6 cars depending on the line, the most common being 5 cars (Line 14 may have 8 cars in the future), but all trains on the same line have the same number of cars.