By the time Robert Friese starts his 8 a.m. shift as a bike train operator for Citi Bike, there’s usually a line of commuters waiting around an empty station in the East Village.

Friese will load his bicycle train, a custom bike-drawn trailer that pulls makeshift trailers with eight Citi Bikes, at a Stuyvesant Town garage and check his app for the stations that “need love”— usually 14th Street and Avenue B or Seventh Street and Avenue A — before heading out.

This is an intense shift, so the 47-year-old packs up bags of nuts, energy bars and a water canteen to help keep him energized as he carries about 500 pounds of bikes to distribute around about a two-mile radius in the neighborhood — all on a cargo bike without a motor to assist him, riding by the same potholes, double-parked cabs and other traffic challenges over and over again.

“I’m a tree hugger, so doing this job for me is like planting seeds,” said Friese, of Williamsburg. “And, you know, there’s a traffic calming element to it — there’s a lot of residential areas and a lot of schools and 20-miles-per-hour school zones. I think the human pace is where it’s at.”

Just after 8 a.m., Friese brought eight bikes to 14th Street and Avenue B to a line of 15 commuters. He apologized and assured that another bike train was on its way.

As Citi Bike continues its second phase of expansion further uptown and deeper into Brooklyn and Queens, bike trains are just one of a variety of organizational approaches for station “rebalancing,” the process of making sure Citi Bike stations remain neither completely empty nor completely full. There are about 150 staffers dedicated to the task that gets most complicated on weekdays, when riders pedal into Manhattan’s central business district each morning and out each evening.

And it’s even more challenging now as the weather warms. A larger network means a greater number of people heading toward Manhattan’s core, creating more stress on the network. Motivate, which operates the service, hasn’t found a way to keep all of its 610 stations properly balanced — even with advance data analysis and a fleet of work vans, it relies heavily on station density to make up for any problem areas.

“I could never get a bike at this station in the morning,” said Isabella Plar, a Brooklyn College student and coffee shop worker, as she pulled a bike out of a dock on Seventh Street and Avenue A on a recent Sunday. “But it was never a big deal because I could always walk to another station nearby.”

Aiding in the effort alongside bike trains are work vans stocked with bikes and deployed around the city. Staff at the 24-hour dispatch center in Motivate’s Sunset Park office monitor a digital station map that forecasts future demand and weather and sends service predictions to teams in the field.

Those teams also include bike valets who help absorb the rush at 14 busy Manhattan stations by corralling bikes alongside stations once they’re full, increasing capacity. Motivate has also turned to its customers to help in the effort with the launch of its “bike angel” program last May. Since then, about 4,000 participants have signed onto the program where they earn rewards like extended memberships, gift cards or unique Citi Bike keys to park or pick up their Citi Bikes at certain stations.

“Rebalancing is one of the biggest challenges of any bike share system, especially in a city like New York where residents don’t all work a traditional 9-5 schedule, and though there is a Central Business District, it’s a huge one and people work in a variety of other neighborhoods as well,” said Citi Bike spokeswoman Dani Simons in an email.

The stresses of expansion have been reflected on worker conditions, according to the union that represents Citi Bike workers, the Transport Workers Union Local 100. That’s evidenced by the dimly lit Delancey Street Depot under the Williamsburg Bridge, where pigeon droppings from the birds roosting under the bridge cover the Citi Bike vehicles and equipment stored there. The union has also complained of a large rat presence in the workplace.

“With the pressures to expand something’s got to give and it’s here,” said Nicholas Bedell, TWU Local 100’s education director. “This was a storage area that was turned into a work area. You can’t ask a worker to spend a day here.”

Simons said that Motivate has scheduled visits for exterminators and added that the union hadn’t previously reached out about the conditions.

“We make every effort to provide good working conditions and be responsive to our employees’ concerns,” Simons said. “We are taking immediate action to address issues at the Delancey facility.”

Since Citi Bike launched in 2013, bike share startups have emerged abroad, with varying degrees of success, offering a different form of “dock-less” service geared toward removing some of these challenges. Through smartphone apps, users can find, lock and unlock bikes anywhere — with no stations required. Some have begun eying launches in New York.

As the city and Motivate negotiate its next expansion, which could bring service to Staten Island and the Bronx for the first time, the de Blasio administration is exploring if Citi Bike could incorporate similar service in its operations. But Motivate believes a dock-less model that allows riders to leave their bikes virtually anywhere has the potential to depreciate service by making rebalancing even more difficult.

“At Citi Bike, in addition to moving bikes by van and truck, we’ve tried to be innovative in how we meet this challenge,” Simons said.