The TGV trains reach their maximum power on 25 kV / 50 Hz, all LGV in France (and Belgium) are equipped like this.
The engines of every TGV are inside the two powercars (one engine per axe; 4 axes in each powercar). The only exception is the TGV-PSE: The two axes of the passenger coach next to every powercar are also driven.

When changing the voltage (i. e. when leaving a normal track and entering a LGV), the train driver receives an information via cab signalisation and loweres all pantographs for a short time.

direct current = all pantographs up
alternating current = one pantograph (the one on the rear powercar) per train up
(the powercars are linked with an alternating current cable on the roof)

All TGV trains can be coupled to so-called 'UM's (French: unite multiplé) with the automatic Scharfenberg-coupler, which means that two TGV units can operate together as a whole.

Another famous TGV technology is the so-called 'articulated trainset', between all trailers (except the powercars). It means that two coaches are 'sharing' one chassis/bogie with 2x 2 wheels. It guarantees comfort and a high stability, especially in case of derailment. Once a TGV derailed with 300 km/h and nothing serious happened! Only some slightly hurt passengers and a few coaches standing parallel to the track. If this would have happened to the ICE without the articulated trainset technology....