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The TGV Duplex is a French high-speed train of the TGV family, operated by SNCF, the French national railway company. It is unique among TGV trains in that it features bi-level carriages. The Duplex inaugurated the third generation of TGV trainsets, and was specially designed to increase capacity on high-speed lines with saturated traffic. With two seating levels and a seating capacity of 545 passengers, the Duplex maximizes the number of passengers carried in one trainset. While the TGV Duplex started as a small component of the TGV fleet, it has become one of the system's main workhorses.
Why the duplex:
When a high-speed line reaches its saturation limit, there are several options available to increase capacity. Perhaps the most obvious way is to reduce train headways (the time between each train), i.e. to fit more trains onto the line at once. To achieve this, increasingly complex signalling systems and high-performance brakes (to reduce braking distance) are required. While these avenues have already been pursued to some extent (headways have been reduced to three minutes on some TGV lines), the technical difficulties of continually improving signalling and braking make other solutions to the problem more attractive.
The LGV Sud-Est from Paris to Lyon is the busiest high-speed line in France, and since its opening in 1981 it has rapidly reached capacity. The mitigation of this problem was to run two trainsets coupled together in multiple-unit (MU) configuration, but even this has not provided enough capacity, and has the added disadvantage of requiring very long station platforms. If you can't make a train longer or wider, then the remaining option is to make it taller; hence the TGV Duplex, with passenger seating on two levels, and 45% more passenger capacity than an equivalent single-level TGV.
Two TGV Duplex sets can run together like all other TGV trains. Multiple formations of one Duplex and one Réseau are commonly seen.
