The TGV Duplex is the third generation of French high-speed train and is the first TGV trainset to feature double-decker carriages. It was specially designed to increase capacity on high-speed lines with saturated traffic and features a seating capacity of 508 passengers.
After its opening in 1981, the LGV Sud-Est from Paris to Lyon rapidly reached capacity. To counter this, the separation between trains was reduced to three minutes on some TGV lines, but the increasingly complex signalling systems and high-performance brakes to reduce braking distance limited this option. Another option is to widen the train but is generally not practicable due to loading gauge restrictions. Running two trainsets coupled together in multiple-unit (MU) configuration provides extra capacity, but required very long station platforms. Given length and width restrictions, the remaining option was to adopt a double-decker configuration, with seating on two levels, adding 45% more passenger capacity. TGV Duplex sets are often run with a single deck Réseau set or another Duplex set.