If you begin to threat model the physical security necessary, you quickly see how asymmetrical it is, regardless of the money thrown at it.

@liotier, I've been rate-limited and cannot reply to you directly, so I'm editing my response into this comment. My apologies.

I didn't intend any slight to French infrastructure or those designing and securing it, just the dynamics of the problem. It is easier to attack than to defend, particularly in physical security. I'm sure critical infrastructure is well taken care of. Do you know if TGV is considered as critical as the other sites you noted? I'm curious of your opinion about what level of expertise and intelligence collection it required to carry this out.