A commuter rail derailment and collision outside New York City during the Friday evening rush hour left 72 injured, with three remaining in critical condition as of Sunday afternoon. About 700 people were on board the Metro-North trains when one heading east from New York City derailed and was hit just outside Bridgeport, Connecticut by a train heading west from New Haven.

Passengers described a terrifying scene, with metal flying and bodies hurled into the air on impact. Lola Oliver of Bridgeport told AP, “All I know was I as in the air, hitting seats, bouncing around, flying down the aisle and finally I came to a stop on one seat. It happened so fast I had no idea what was going on.” Photos at the scene showed cars torn open and crushed, with twisted metal and debris scattered over 200 yards.

Metropolitan Transportation Authority (MTA) spokesman Aaron Donovan told the media, “The eastbound train derailed, which is what caused the trains to collide. It derailed in such a way that it went into a path of the westbound train on an adjacent track.” Witnesses told the Hartford Courant that they felt a sharp bump, then saw a cloud of smoke and dust as the trains jumped the tracks.

Most of the injured were in two trains where the impact occurred, the third car on one train and the lead car on the other. Donovan said damage was “significant to both tracks and overhead wires” and that it would “take a substantial effort to repair.”

Metro-North is the busiest commuter rail line in the US in terms of passenger volume, with some 125,000 people travelling each day in the affected region. The Metro-North main lines run northward out of Grand Central Station into suburban New York and Connecticut. The line will remain suspended between South Norwalk and New Haven, Connecticut for days, as crews rebuild 2,000 feet of track, overhead wires and signals damaged in the crash.

Authorities were predicting a nightmarish commute Monday morning, as thousands of commuters were expected to drive on the already clogged roads from the affected area into New York City. Amtrak service in the Northeast Corridor between Boston and New York has been suspended indefinitely. Neither the MTA, which operates Metro-North, nor Amtrak would say when service would resume. Bridgeport Mayor Bill Finch said the disruption in train service could cost the region’s economy millions of dollars.

On the scene Saturday, National Transportation Safety Board (NTSB) member Earl Weener said a fractured section of rail is being studied and would be sent to a lab for analysis. It is unclear whether the rail was damaged before the collision, or as a result of it. Weener stated it was too early to speculate on the cause of the crash, and that authorities had no reason at this point to assume that foul play was involved.

Federal investigators will also look at the brakes and performance of the trains. Both vehicles involved were new M8 trains, the commuter line’s most advanced, which were put into operation in early 2011 after several delays, replacing some trains that were decades old. Investigators will also look at the condition of the tracks, as well as crew performance and information on signaling. Weener said the NTSB had downloaded reams of data to be analyzed.

The damaged train cars are likely to remain at the scene of the crash for days, until the NTSB completes its on-site investigation. MTA spokesman Donovan stated Saturday, “When we get approval from NTSB, we’re going to have to have a crane come in and lift the damaged train cars.”

While Connecticut Governor Danell Malloy said he hoped the NTSB could conclude its on-scene probe on Monday, other sources said it might take up to 10 days. Once the initial NTSB investigation is completed, the tracks will be handed back to the MTA to begin the repairs. Cranes and trucks will be brought in to remove about 15 cars and then the tracks have to be rebuilt.

According to the NTSB, about 1,000 people die in train accidents every year in the US. These accidents include train-on-train collisions, trains colliding with other vehicles, and trains hitting individuals. Statistics show that train derailments leading to chemical and other hazardous spills occur almost every two weeks.