SMART officials said Wednesday they are close, but not yet ready to have federal rail inspectors sign off on the commuter rail system to begin operations.

The fledgling system is working to synchronize a complex array of computerized warning and train control systems as it readies for passenger service.

The Sonoma-Marin Area Rail Transit agency has spent $50 million on implementation of what is known as a “Positive Train Control” system on the line from downtown San Rafael to the Santa Rosa Airport. The system essentially controls movements on the rails electronically to slow or stop trains before certain types of accidents involving speed could occur.

SMART is poised to be the first stand-alone rail line system in the country to start service completely outfitted with a safety system.

That system is being tested.

“We are forcing the system to fail,” said Farhad Mansourian, who heads SMART.

So far, the system has worked more than 97 percent of the time in 900-plus tests. When it has failed, the system still stopped the train, but a few seconds later than it was supposed to, Mansourian reported to the SMART board Wednesday.

SMART also continues to test its 63 rail crossings — where vehicle traffic crosses tracks. By comparison the Bay Area Rapid Transit system has zero crossings.

“We have finalized all but eight of the crossings,” Mansourian said. “One hundred percent is what we are looking for. We think that should be done by the end of April.”

Mansourian is sticking with a starting time frame of later this spring for service. But an exact date has not been set. And before any trains carry passengers, the Federal Railroad Administration will have to sign off on the project.

“Only at that time, when we think all testing is done, is when we will then ask the FRA to come in to audit us,” said Mansourian, who expects to have a better time frame for when that will occur later this month. “We don’t want them to come in and say, ‘you’re not ready.’”

Marin Supervisor Kate Sears, a rail board member, praised the testing.

“It’s great we are being as tough as we are, as strict as we are,” she said.

Jake Mackenzie, a rail board member from Rohnert Park, said he has heard the public’s cry pushing for the start of service. Voters approved the rail plan in 2008.

“People ask, ‘how come we have not started service?’” he said. “It’s important for people to realize we fall under the jurisdiction of the FRA. We operate under regulations. There is a final write off, and that is with the FRA.”

Said Mansourian: “It’s complex putting a train into service. … We are in a very good place, but we are not satisfied because we are not at 100 percent. We cannot operate until we are 100 percent safe.”

Meanwhile, SMART has now accepted all of its trains from the manufacturer after engines were replaced over the past five months. The discovery of a design flaw with the crank shaft in the engines required all 14 SMART engines to be rebuilt by the manufacturer, Indiana-based Cummins Inc.

“They are ready to go,” Mansourian said.