MBTA rail inspectors are stepping up their response to the thousands of crumbling concrete ties on commuter lines, ordering engineers to throttle back for safety along the damaged tracks.

It is the latest problem with defective ties along the Old Colony commuter Rail lines south of Boston, where 11,000 passengers ride the trains into Boston every day on the two lines starting in Middleborough and Kingston.

The so-called “slow orders” — reducing speeds from 70 to 30 miles per hour — were placed beginning last Friday on trains running on the Kingston line because of an increasing number of broken, cracked, and crumbling concrete ties that present a potential danger if they are not replaced or reinforced.

The T last week started a shutdown along a stretch on the Middleborough line in non-peak hours until the end of October to allow workers to take out 14,000 defective concrete ties and replace them with wooden ones.

“The rule is when we see four consecutive ties [with cracks or other defects], we issue a slow order until they’re either replaced or wooden ones are put in to [strengthen] them,” said one inspector, who did not want to be identified because he is not authorized to speak. “Everybody is concerned with safety for passengers. That’s our biggest thing.”

T spokesman Joe Pesaturo acknowledged the speed was reduced to 30 miles per hour last Friday on three sections along a seven-mile stretch of the Old Colony line between Hanson and Kingston while workers insert wooden ties. But while Pesaturo insisted the slow orders would be lifted Monday, trains continued to slow down approaching and traveling through the potentially unsafe areas Tuesday. Bright green, yellow, and white paint used by inspectors to identify crumbling concrete ties in various stages of distress are visible through long remote stretches of the line between Halifax and Kingston.

Scores of new wooden ties lay along the side of the tracks waiting to be inserted, while at a staging area between Weymouth and Abington, hundreds of wooden ties had been dropped in bundles to be used in the maintenance efforts.

The issue of the defective concrete ties continues to vex T officials. In its Summer issue, CommonWealth magazine reported that the growing problem could affect as many as 150,000 concrete ties on the Old Colony line purchased from Rocla Concrete Ties of Denver and made at the company’s Bear, Delaware, plant.

It appears the issue involves a defective mix of concrete that is vulnerable to the freezing and thawing conditions in the Northeastern United States. The 800-pound ties carry a 25-year warranty but started showing cracks after less than a decade of use. Similar problems have occurred with Amtrak and the Long Island Railroad, with each of the systems replacing hundreds of defective ties purchased from Rocla at a cost of several hundred million dollars despite the 25-year warranty.

At issue is train safety because consecutive faulty ties that are not replaced or strengthened with additional wooden ties could cause a shifting of the tracks and a possible derailment. An accident in Washington state several years ago that caused a train to derail and injure passengers was triggered by 19 consecutive cracked and broken ties that were spotted but not removed or strengthened by rail workers because of a lack of regulations specific to concrete ties, according to the National Transportation Safety Board.

T officials have told CommonWealth that Rocla, whose officials have not returned repeated calls from the magazine, has threatened bankruptcy if it is forced to honor the warranty. So far, rail workers have identified more than 7,000 broken ties, with more being spotted every day, and more than 15,000 are being removed, replaced, and reinforced with wooden ties by the Massachusetts Bay Commuter Railroad company, the for-profit operator of the state’s commuter rail lines.

Some T officials have avoided requests for comment and questions at public meetings, while Pesaturo has downplayed the increasing problems and insisted there are no safety issues.

“There have been a few isolated speed restrictions in the past to maintain compliance with MBTA maintenance standards while crews addressed any tie issues,” Pesaturo wrote in an email for the original story. “These restrictions were removed as soon as the tie conditions were addressed.”

The problems are occurring as officials have decided to shut down the section from Middleborough to Bridgewater on the Old Colony Commuter Rail between peak commuting hours (that is, from 8:30 a.m. to 4 p.m.), at least through the end of next month to remove hundreds of defective concrete ties and replace them with wooden ones. Passengers are being bused from Middleborough to Bridgewater, with the buses leaving 20 minutes earlier than the scheduled train.

While the problems on the Kingston section of the Old Colony are not as widespread, they are beginning to grow as inspectors walk the lines and identify cracked, crumbling and broken concrete ties.

While MBTA officials are denying any other problems, inspectors along the Greenbush line are beginning to spot similar problems with faulty concrete ties on the relatively new rail line.

“No such issues on the Greenbush Line, and no speed restrictions on the Greenbush Line,” Pesaturo wrote.

But a scrolling message on electronic boards at station stops on Monday informed passengers of five- to 10-minute delays because of ongoing track work.

“There’s a few [faulty ties], not near as many [as Old Colony],” said the inspector.

In addition to the potential safety issues, the breaking ties and the work to replace them are causing major disruptions and minor irritants for commuters south of Boston. The changes are affecting other lines as trains are forced to wait for other trains to pass crossings and single-track sections during times they normally would not encounter each other because of staggered schedules.