NBC Bay Area has learned that a failure on the coupler system used on dozens of new Muni trains has prompted the San Francisco transportation agency to order inspections and new restrictions.

Until further notice, the fleet of new train cars can only run individually, meaning they cannot be combined to make longer trains.

On the same day the coupler system failed, April 12, a woman fell into the trackway when she got her fingers caught in a door of another Muni train, raising questions about the single-door system's failsafe mechanism.

Muni officials say they are dealing with both issues.

While 68 new rails cars are being inspected Muni will shift around its older model cars to fill in service gaps. So far, inspections have found at least one other failed coupler. Had that coupler failed during regular service, the result could have been far worse.

Muni spokesman Paul Rose said the investigation is focusing on the so-called shear pin, which fits into the coupler and links the trains physically and also allows electric current to flow to power the second rail car.

"It allows them both to operate as a unit," Rose said.

Rose said in the case of the failed coupler, the two-car train had finished its Friday operations when the driver discovered the problem.

"It was at the end of service – there was a loud popping noise," Rose said.

The train had come to stop and so did not separate when the pin failed. Rose says that by design, the pin is supposed to fail on impact, allowing another system to kick in and absorb the shock of a collision.

Rose also said that additional inspections found another failed shear pin from another train that had also had finished service for the day.

"We are not putting (two car new trains) out on the street until we find out what the root cause is and understand the failures," Rose said.

Also on Friday, he said, a woman got her fingers caught in the single door of a train at the Embarcadero station.

"We had to stop service because she fell into the trackway," Rose said.

The woman was taken to the hospital, and is expected to recover, Rose said. The woman was apparently chasing the train and put her hand in to stop the door from closing, he said.

"We know that it appears the person was trying to chase the door as the train was leaving the station," Rose said. "We encourage people not to chase the train and not to stop the doors with their hand."

He says while the door does have technology to guard against such accidents, Muni has installed another sensor on the edge of door of some trains to provide "an additional layer of protection."

Muni critics believe the system also has several other issues. Some have complained about jerky, uneven movements during operation. Also bench seats, designed to accommodate large crowds, have drawn complaints about passengers sliding out of them, a problem Muni says is being dealt with.

Rose says the agency is also investigating reports of incidents involving doors that date to October and December.

The coupler failure comes as another 150 new cars are currently on order from the same manufacturer, Siemens. The city is in the process of putting up another $62 million to rush delivery.

The president of the watchdog group Save Muni, Bob Feinbaum, said operators have told his group about trouble coupling cars for the last few months.

"We’re asking for them to hold the procurement of the new 150 vehicles," Feinbaum said, until there’s proof that four car trains can be safely and properly coupled together.

"This failure shows that they can’t be and they should not be ordering more trains until they can show that they be coupled,” Feinbaum said. He then added that the key is the trains need to be joined together and separated not only at the yard but during service to better handle downtown crowds.

While he has not heard about the door issues, Feinbaum said disabled passengers have long complained about getting through the new car doors, which he believes are slightly narrower than doors on the old fleet.

"The doors are supposed to open level with the platform,"’ he said, "and we hear that they don’t, for some reason."

City Supervisor Aaron Peskin, who heads the transportation oversight committee on the board, told NBC Bay Area that he was informed about the problems on Wednesday by Muni officials. He said he had been assured by the head of Muni, just last week, that the coupler problem had been dealt with.

"Obviously, it hasn’t – they are running one car trains," Peskin said. "The fact that they are taking this seriously and they are looking at all the couplings and all their trains is a good thing. The fact that they’ve been acquiring new light rail vehicles that seem to be experiencing performance failures is deeply troubling."