BNSF doesn't have to sue anybody. As I understand the standard Amtrak contract, Amtrak is responsible to the railroad for all damage related to a crash involving a passenger train regardless of who caused it. Standard hold-harmless kind of thing. It's up to Amtrak to try and squeeze a few cents from the truck owner and/or operator. In a case like this, Amtrak (or Amtrak's insurance) will probably eat most of the cost because the truck people don't have any resources - you wanna junk truck? - and if they have insurance it won't come near covering the costs. It's a cost of doing passenger railroad business.