Brandon Bostian's phone records show calls were made, text messages were sent and data was used the day of the crash, the National Transportation Safety Board said, but it remains unclear if the phone was used while the train was in motion.
Investigators won't be able to make that determination until after a time-consuming analysis comparing time stamps from Bostian's subpoenaed phone records with those from an on-board data recorder, video and other sources, the NTSB said.
The May 12 derailment killed eight people and injured more than 200.
Investigators are looking into why the train from Washington to New York City was going double the 50 mph limit around a sharp curve about 10 minutes after leaving Philadelphia's 30th Street Station.
Bostian's lawyer, Robert Goggin, has said the engineer kept his cellphone in a bag and used it only to call 911 after the derailment. Bostian, who was injured, told investigators he had no recollection of the crash, the NTSB said.
"The next thing he recalls is being thrown around, coming to, finding his bag, getting his cellphone and dialing 911," Goggin told ABC News the day after the crash.
Goggin has not returned repeated messages from The Associated Press.
Federal rules prohibit engineers from using cellphones while operating the train or preparing for movement.
The Federal Railroad Administration issued an emergency order and later adopted a rule banning electronic devices after a 2008 crash near Chatsworth, California in which investigators said an engineer ran through a stop signal while texting a friend.
The Metrolink commuter train hit an oncoming freight train, killing 25 people and injuring 135 others.
Railroad Workers United, a consortium of train unions, said Wednesday that Bostian's shift the day of the derailment had been particularly grueling and that equipment-related delays on his earlier train to Washington shortened his rest break.
A system that displays track signals on the train's dashboard failed, forcing the 32-year-old Bostian to pay close attention while reducing speeds on the Acela Express train — which tops out at 150 mph in designated areas — to below 80 mph, the organization's Ron Kaminkow said.
"It wasn't a routine run," Kaminkow said.
The Acela arrived at Washington's Union Station 26 minutes late, leaving Bostian about an hour to rest, eat and go to the bathroom before his trip back to New York on the Northeast Regional train that eventually derailed in Philadelphia, according to Karl Edler, a veteran Amtrak engineer with knowledge of Bostian's schedule.
Engineers used to have at least 90 minutes between trips, Kaminkow and Edler said, but a March 23 schedule change ended the decades-old practice. The swift turnarounds have "the ability to create more fatigue in the workforce, plain and simple," Kaminkow said.
Amtrak spokesman Craig Schulz said the railroad revisits crew assignments periodically to look for efficiencies or reflect changes in train operations and that the recent changes were tested to ensure they meet federal safety regulations.
"We believe these schedules address quality-of-life issues for our crews and are better than the schedules that had been in place for many years," Schulz said.
Bostian talked to investigators May 15 and did not report feeling fatigued or ill prior to the derailment, according to the NTSB. Edler has not spoken with Bostian, but said his statement to investigators with a lawyer present may not have reflected how he truly felt.
Edler — the leader of a Brotherhood of Locomotive Engineers and Trainmen local in Washington, D.C., and a former member of the union's safety task force — said he did not speak for the 52,300-member organization.
The union issued a statement Tuesday urging Amtrak to add a second crew member to train locomotives and criticizing Congress for cutting funding to the railroad.
Bostian had been an engineer on the Northeast Corridor for about three years. He was based in New York. He was specifically assigned to the Washington to New York route for several weeks before the derailment, the NTSB said.
He worked a five-day-a-week schedule — making a daily roundtrip from New York to Washington — and had a "very good working knowledge" of the territory and various speed restrictions, according to NTSB Board Member Robert Sumwalt.
Associated Press writer Maryclaire Dale in Philadelphia contributed to this report.