With one hand on the control stick, Lisa Callahan waits behind a blinking dashboard for the vertical “go” signal to flash from the tracks outside Union Depot in St. Paul.

When it does, she double-checks that the road is clear, radios the control center to confirm she’s ready and pushes the stick toward acceleration.

With that, the 48-minute journey of the Green Line light rail has begun, taking riders from Union Depot through the heart of the University of Minnesota campus to Target Field.

Callahan, one of 131 light rail operators for Metro Transit’s Green Line, has been driving light rail trains for four years. She started as a day care provider but shifted to a career in transportation that has lasted nearly two decades.

“It’s the greatest job ever,” said Callahan, who spent 14 years driving buses before switching to trains.

When Callahan begins her day at Union Depot, she checks in with a facial recognition system that allows the control center to determine if she looks fit for duty. Light rail operators work eight-hour shifts five days a week — at all hours of the day and night.

Back on the train, Callahan must maintain her focus as she rolls through the Twin Cities. Operators control the entire process of running a light rail train, from acceleration and deceleration speeds to the timing of door openings and closings.

To prepare for this responsibility, employees are required to have bus-driving experience, said Christina House, the Metropolitan Council’s rail coordinator.

Training totals six weeks, House said, consisting of two weeks in a classroom with tests and a final exam, followed by four weeks of field training.

Once in the field, she said, trained supervisors accompany operators to make sure trainees feel comfortable behind the controls before they go it alone.

Some decide against completing the training, House said, out of concern they won’t be able to maintain the necessary focus or deal with stress-inducing situations.

“It’s not for everyone,” she said.

While shut off from the rest of the world in the conductor’s pit, Callahan trekked up University Avenue West, carefully observing each train speed-limit sign, watching for changing stop signals and ringing a high-volume bell at any inkling of danger along the way.

She said she doesn’t get tired of the job, except for the worst part — the constant stress of watching for irresponsible drivers and pedestrians.

“They’re always looking [for pedestrians],” House said.

Callahan chuckled in response. “Tell me about it,” she said.

Nearing the next station, Callahan eyed the yellow band lining the platform’s edge. She checked for unwitting passengers as she pulled up to a stop and tooted the rail horn, as she always does if necessary to warn of her approach.

Before she left, she monitored video mirrors to watch for stragglers on the platform. She has a duty to maintain a schedule while accommodating all passengers.

“As an operator, if I see [elderly passengers] on the platform, I will wait,” Callahan said, adding that she’ll take off from the platform slower, too, allowing the older population extra time to take their seats.

As the train whizzed to and from each stop, Callahan remained calm — even when a car made a U-turn in front of the train near the Snelling Avenue station.

Braking as fast as possible and sounding her horn before coming to a halt, she narrowly avoided a collision. Immediately, she reported the near-accident to the control center.

“They got to be tough to get through stuff like that,” House said of the drivers. “There’s a lot coming at these operators really fast.”

Moving into Minneapolis, Callahan stopped at the University of Minnesota’s East Bank station — the most popular boarding spot on the Green Line, said Metro Transit spokesman Drew Kerr.

The line has topped 1 million rides since September, he said, blowing 2015 projections out of the water by 15 percent.

For Callahan, the numbers are encouraging.

“Knowing that we’re breaking records on picking up people and getting to their destination on time is great,” she said.

Past the University campus, Callahan wove through downtown Minneapolis, almost at the end of her journey.

Like the University stop, the downtown Nicollet Mall station was full. It’s the second-most popular stop on the line, Kerr said.

At Target Field station, Callahan opened the doors and turned the key to the off position. Her first ride of the day was over, but she covered the same ground five more times before heading home.

She stepped off the train to stretch her legs, but a smile never left her face.

“I really enjoy the Green Line. I think it’s a challenging line with all the customers, pedestrians and vehicles,” she said. “I like the challenge.”