I don't know whether they were referring to intercity passenger rail (Amtrak) with this, but what's rather surprising is that now Amtrak, at least in North Carolina, has just about zero security. You line up on the platform, you show the conductor your ticket when you get on the train (or say that you need to buy one), and then there's a random chance that your ID will be checked after you sit down and the train gets underway (mine never has been). Checked baggage is probably checked (not sure), but they don't even run your carry-ons through an X-ray scanner or ask you to walk through a metal detector.

At most stations the platform is at least locked most of the time (even if it's only with a gate and a chain-link fence), but at some (like Raleigh) anyone can walk up to the platform at any time. Yet despite all this I can't recall ever hearing of a violent act of terrorism being committed on a train.