You can't really guard thousands of kilometers of rails, or have much redundancy considering the cost of the infrastructure. The incesseant budget cuts and progressive privatization certainly do not help.
@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.
I can throw burning fuel into a manhole or at a street cabinet, and it doesn't take much insight to find one that carries important traffic - take your pick near a military base or a nuclear power plant. But the attack will be ineffective, because the attacker needs to know what other bits of infrastructure carry the backup circuits - and take them out simultaneously. And, of course, for the important stuff such information is classified. So, an effective attack on French critical infrastructure is more difficult than it seems at first glance.