Attaching cargo as in adding another car or tanker? That would be pretty difficult - those movements are planned. Sticking a bomb on a tanker, on the other ha

Attaching cargo as in adding another car or tanker? That would be pretty difficult - those movements are planned. Sticking a bomb on a tanker, on the other hand, would probably be much easier, although you'd have to know beforehand what tankers are full, and I suspect that's more difficult. But there's always going to be a level of insecurity around trains. A train yard can't be fully fenced off for obvious reasons. I know here in Oslo there was a teenager killed last year after he and a couple of others snuck into a yard one night and went into a maintenance tunnel and touched a high voltage line. They've rebuilt the fence they got in through, and made it higher, but someone determined to get in there will just go further to where the tracks cross a road.