Some years ago I wrote a conference paper [0] pointing to the clear difference in security screening for a flight from Lyon to Paris, and the same journey taken by TGV. The train would have more people on board but no screening of passengers, and only a friendly warning that you had to have a ticket to ride the train (in practice, you could buy one on the way or not buy one at all if you got lucky). The ticket could be bought from a machine or counter without ever showing an ID, and anyone could load baggage onto the train even without later boarding. It seemed to me to be an obvious target for a terrorist who wanted to cause some havoc in the French transport sector, and a compelling one now that the flights were so locked down and difficult to board with ill intentions.

0: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=1621509