The TGV Duplex is a French high-speed train of the TGV family, manufactured by Alstom, and operated by the French national railway company SNCF. They were the first TGV trainsets to use bi-level passenger carriages with a seating capacity of 508 passengers, increasing capacity on busy high-speed lines. While the TGV Duplex started as a small component of the TGV fleet, it has become one of the system's workhorses.

A total of 160 Duplex trainsets were built: the original order of 89 first constructed in 1995, an additional 19 Réseau Duplex trainsets created as an extension of the TGV POS project in 2006, and 52 second-generation Dasye trainsets were first delivered in 2007 with revised traction motors and safety systems.

The Duplex design was further refined into the third generation Euroduplex.

The LGV Sud-Est from Paris to Lyon is the busiest high-speed line in France.^{[citation needed]} After its opening in 1981 it rapidly reached capacity. Several options were available to increase capacity. 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) required, 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 is to adopt a bi-level 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.^{[citation needed]}

The Duplex feasibility study was completed in 1987. In 1988, a full-scale mockup was built to gauge customer reactions to the bi-level concept, traditionally associated with commuter and regional rail rather than with high-speed intercity trains. A TGV Sud-Est trailer was tested in revenue service with the inside furnished to simulate the lower floor of a bi-level arrangement, and later that year another TGV Sud-Est was modified to study the dynamic behavior of a train with a higher center of gravity. Discussions with GEC-Alstom began soon after, and in July 1990 the company won the contract to build the "TGV-2N", as it was then known. The contract was finalized in early 1991, at which point the official order was made. The first tests of a bi-level trainset were in November 1994. Soon after their first run, the first rake of eight trailers was tested at 290 km/h (180 mph) on the Sud-Est line. The trainset was powered by TGV Réseau power cars at the time, as the Duplex power cars were not ready. The first Duplex power car was mated to the bi-level trailers on 21 June 1995.

In 2007, a shortened TGV Duplex trainset equipped with distributed traction technology, called the AGV, set a rail speed record of 574.8 km/h (357.2 mph) during test runs.

The Duplex design introduced a number of technical changes relative to earlier single-level TGV trainsets. A comparison between the original TGV Sud-Est and a Duplex trainset illustrates differences in power-to-weight ratio and weight per seat:

In this comparison, "power" refers to installed power rather than the power typically used during operation.