At the power changeover location on LGV MED , you take the pantos down for that 500 metres , meaning the overhead lines are pointless , so what is the point of them being there ? In case the train brakes down ? If anyone knows feel free to share your answer
Maybe and that's really just a maybe. They are still there in case it happens when a train accidentally stops. So that the train can save itself from the situation and does not have to be towed
The actual neutral section is relatively short, maybe just between a couple of masts. The TGV also swaps pantographs from one connected to the AC Transformer to one connected directly to the traction packs at this point. Given that having the 1500V pan contact 25kV would cause a very bad day for all and probably result in a visit from le Pompiers they build in a fair margin for error in requiring the pans lowered well in advance - that said changeover errors do happen more regular than you'd hope. Some other systems use the same pantograph for AC/DC or different AC voltages and the pan stays up throughout - trackside beacons open and close the breakers as appropriate. This happens in Japan at the AC/DC boundary north of Tokyo, as well as on the Horrioko Shinkansen where it crosses the 50/60Hz boundary between power grids. It's also common with tram trains, certainly the newer German systems have automatic changeover where they leave street running (~750V DC) and join the 15kV mainline
I don't 100% know why but this seems very logical: I think it's a saftey measure because if anything goes wrong the TGV has some protection against the wrong power input and then can be put on a powered section with outside help again or just by switching cabs i guess, but if there were no cables there the pantograph would shoot straight-up (the pantograph puts a lot more force on the cables than you think) and would then cause a lot of expensive damages on not only the TGV itself but on upcoming cables and catenery.
Spot on and for the TGV Duplex in the 15kv 16.7hz capable Euroduplex the rear 1.5kv DC 1.95m pantograph also feeds the TGV Euroduplex and an earlier version Thalys PBKA power for 15kv 16.7hz Germany. If you want the model train of one just look at marklin 37793 and see that the rear DC Pantograph is up for 15kv 16.7hz HMA and Appenweier-Strasbourg Railway where it's dropped for 1.45m 25kv pantograph at the Rhine River Bridge
