"The public would never accept an airline operation with a single person in the cockpit," the Brotherhood of Locomotive Engineers and Trainmen said in a statement. "There is no reason that rail employees and rail passengers' lives should be viewed any differently."
Brandon Bostian, 32, was alone in the locomotive of Train 188 when it derailed May 12, about 10 minutes after departing Philadelphia for New York.
Train 188 had a five-member crew — including a conductor who is still hospitalized with serious injuries — but they were in the passenger coaches, closed off from the locomotive.
Amtrak hasn't had a second crew member in the locomotive of its Northeast Corridor trains since Congress ended the requirement in the early 1980s, the union said.
In a statement, Amtrak said it operates its locomotives in accordance with federal standards and that its locomotive engineers and train conductors work together to ensure safe train operation.
"Amtrak is and always has been committed to safety as its number one priority and continues to work with all the stakeholders toward a consensus on the appropriate management of crew resources in passenger train locomotives," the statement said.
The National Transportation Safety Board is looking into why the train was going more than double the 50 mph limit around a sharp curve. It is also investigating whether an object may have struck the locomotive's windshield before the crash.
The FBI said it found no evidence a grapefruit-sized fracture on the windshield was caused by a firearm, and the NTSB said it was unsure anything had struck the vehicle.
Investigators said Bostian, who was among the injured, told them in an interview that he couldn't recall anything from the last 3 miles before the derailment.
About 20 people remained hospitalized Tuesday. Five were listed in critical condition.
Authorities say it could be a year before they determine the probable cause of the derailment.
The union Tuesday called on Congress to require a second "fully trained and qualified" crew member in each locomotive, but stopped short of demanding that person also be an engineer.
Sen. Chuck Schumer, D-N.Y., suggested Tuesday that Amtrak at least place a second crew member in the locomotive until automatic speed control technology is fully implemented.
A former NTSB chief railroad crash investigator questioned the idea, saying putting multiple crew members in a locomotive was "more of a distraction" than a safeguard and most likely would not have prevented the May 12 derailment.
The former investigator, Ed Dobranetski, pointed to a 1996 collision outside Washington, D.C., in which a commuter train engineer was thought to have been distracted by a conversation with a crew member riding along in the cab compartment, causing a crash with an Amtrak train that killed 11 people.
Amtrak occasionally employs a second engineer on long-haul routes, according to Dobranetski.
On some railroads a conductor or brakeman will sit to the left of the engineer. With only one set of controls to operate the train, their role is limited to calling out signals and hitting the emergency brake if trouble arises, he said.
"I've done accident investigations where either the conductor is asleep, inattentive or intimidated by the engineer, but 99 per cent of the time the conductor only reminds the engineer of the speed or a situation. He's rarely prevented something," Dobranetski said.
The engineers' union on Tuesday also blamed lawmakers for deep funding cuts it said translated into fewer Amtrak crew members and greater lag time in the implementation of technology that slows speeding trains.
The House Transportation and Infrastructure Committee will hold a hearing on the derailment after Memorial Day. A spokesman said it was not clear if the two-person crew proposal would be discussed.
The hospitalized conductor, Emilio Fonseca, has sued Amtrak, claiming the railroad was negligent and careless. His lawyer said Tuesday he suffered a broken neck, broken back and other serious injuries.
The attorney, Bruce Nagel, said Fonseca, 33, of Kearny, New Jersey, was in a bathroom in the first car of the train at the time of the derailment and that his location likely saved his life.
Fonseca managed to get out of the train and remembers telling people to watch out for electrical wires, Nagel said. Doctors have dubbed him the "miracle man" for surviving the crash.
"Obviously the hand of God was on his shoulder," Nagel said.
Associated Press writers David Porter in Roseland, New Jersey, and Joan Lowy in Washington contributed to this report.