Crews have worked around the clock since Saturday and track rebuilding has progressed quickly, officials with the Metro-North railroad said.
Resumption of service is "tremendously good news," Connecticut Gov. Dannel P. Malloy said at a news conference. Amtrak was also expected to be running Wednesday, state officials said.
The crash in Fairfield at rush hour Friday evening injured 72 people, including one who remained in critical condition Monday. It snarled commutes for roughly 30,000 people who normally use the train in the densely populated suburbs north of New York City and into Connecticut. Investigators are looking at a broken section of rail to see if it is connected to the derailment and collision. Officials said it wasn't clear if the rail was broken in the crash or earlier.
Commuters struggled Monday to find transportation, and many commuters reported rides to work and back home far longer than on typical days. Some used a jury-rigged alternative to the trains: A shuttle train ran between New Haven and Bridgeport, where a bus connection to Stamford circumvented the accident scene, and finally customers boarded a train for New York. That system was to continue Tuesday.
For Gary Maddin, the drive from his home in Milford to the Bridgeport train station normally takes 20 minutes. On Monday, it took an hour. Then he had a shuttle bus and a train ride before he got to his destination, Grand Central Terminal in New York.
"It's a lot," he said. "It's a nightmare just to get into the city today."
Others drove themselves, and state officials nervously watched heavy traffic on two major arteries in southwest Connecticut, Interstate 95 and the Merritt Parkway.
But transportation officials were pleased that area highways were not as choked as they feared, Malloy said. He said commuters heeded his warning over the weekend about the prospect of highways becoming parking lots if all 30,000 of the usual train riders drove instead.
Backups on the Merritt Parkway were less than on an average Monday and I-95 was only slightly more jammed than usual because of fog, Malloy said.
"Today went exceedingly well," the governor said. "People listened to us. Many people stayed home or worked from home."
Federal investigators arrived Saturday and were expected to be on site for seven to 10 days. While they have revealed little beyond their interest in one particular rail section, Connecticut lawmakers say they plan hearings on the crash on a rail network in need of extensive improvements.
Members of the General Assembly's Transportation Committee said they have been briefed by state transportation officials over the years about the hefty investment Connecticut needs to make to fully upgrade the commuter rail line, including a couple of 100-year-old bridges that need to be replaced.
"It's like anything else, you know," said Rep. Tony Guerrera, co-chairman of the committee. "You can have a brand-new car and it runs great, but if the roads are awful, with potholes going up and down, what good is it?"
The last major collision involving Metro-North occurred in 1988 when a train engineer was killed in Mount Vernon, N.Y., when one train empty of passengers rear-ended another, railroad officials said.
Associated Press writers Susan Haigh and Stephen Singer in Hartford contributed to this report.