All trains, which include information about the sender, the receiver and the route of travel, go through a thorough inspection with the Customs Border Patrol (CBP) before they are allowed to enter the U.S. UU. Because freight train crews are already authorized by U.S. Customs, there is generally no need to stop unless requested.
Passenger trains are a completely different scenario, since customs has no idea who is on the train until all the passengers are processed. I haven't brought a train since 2003, but apparently trains are now also scanned electronically (X-ray machine).
