MBTA commuter rail officials want to create a “ring of steel” of ticketed gates at Boston’s busiest stations to cut down on an estimated $35 million the system hemorrhages each year to fare evaders, a move one watchdog called bold but risky if it slows the flow of thousands of rush-hour commuters.

“It’s a partial solution to a very, very large problem,” Greg Sullivan, the state’s former inspector general, said of plans by Keolis to crack down on fare evasion. “The challenge is how to get that mass of rush-hour commuters through turnstiles without creating an epic backup.

“I think they’re heading in the right direction by trying to come up with a solution,” added Sullivan, now of the Pioneer Institute and a daily commuter rail rider himself. “But it’s going to be a challenge.”

Keolis said it estimates that the system loses $35 million annually after doing a survey of more than 1,600 riders, finding that as many as one in five are either underpaying or skipping fares altogether thanks to a variety of tactics.

Among them, officials say, are “short-zoning,” where a commuter pays for a ticket to one stop but actually goes farther down the line; using screen grabs of electronic tickets on their smartphones; or simply moving seats to avoid fare-collecting conductors.

Commuter rail evaders aren’t the only scofflaws cheating the taxpayers. MBTA officials said they are losing as much as $4.5 million a year on the Green Line due to fare evasion and up to $2.4 million in its bus system, pushing the total potential taxpayer tab to nearly ?$42 million annually.

To address it, Keolis officials say they’re willing to pay $10 million for infrastructure upgrades, including a system of ?automatic ticket gates at Back Bay, South Station and North Station, forming what they dubbed a “ring of steel” at stations that ?account for 90 percent of the system’s inbound and outbound traffic.

Combined with other?reforms, including doing more surveys to gauge where fares are going unpaid the most, Keolis officials say they could recover up to $24 million a year, a significant chunk for a system that is projected to produce $215 million in revenue this fiscal year.

Officials cautioned that any plan still needs to be approved, but Brian Shortsleeve, the T’s chief administrator, said if the T and Keolis move ahead, “We would look for Keolis to put up the capital.”

“There is room for improvement. (But) it’s not by adding more people, more staff on board,” said Frank DuBourdieu, Keolis’ deputy general manager.

“This is going to change dramatically the way fares are collected. So the point … is we have the first survey ?ever (on fare evasion). Now we have a baseline on what is the situation. We know, according to our experience, there are ?solutions.”