The 40 new locomotives, which cost $222 million, were brought into service gradually in 2014 and 2015. In the fall, when the majority were finally in service and being used more frequently, MBTA workers struggled with delays on at least one train set led by the new locomotives on the majority of weekdays, when commuters count on the service the most.

A mix of mechanical failures, operator errors, and other problems has led to multiple delays with nearly every new commuter rail locomotive recently put into service by the Massachusetts Bay Transportation Authority, according to a Globe analysis of the rail service's performance from June to December 2015.


The new locomotives are more reliable than the system's 50 older trains and have helped to drive down delays overall, but it's clear bugs still exist: The MBTA is working with its manufacturers to fix at least nine defects on the locomotives while they're still under a two-year warranty, and it may need to fix even more.

Two of the trains have had such troubles with a frozen engine and overheating in the propulsion systems in January that they are being sent back to an Erie, Pa., facility owned by General Electric Co., for tests. GE supplied the engines for the locomotives, which were assembled by Motive Power Inc.

In addition, workers were told to keep the locomotives running overnight because they feared mechanical issues with a battery charger could keep them from restarting in the morning, according to MBTA officials.

"We've known that there have been issues with these locomotives since we've purchased them," said Thomas Murray, president of the union that represents workers who clean the coaches. "It's going to take a lot of money and man hours to make them road-worthy."

Frank DePaola, general manager of the MBTA, said many of the defects were only discovered once the locomotives were being heavily used. The authority, DePaola said, is aggressively tackling the problem by asking Motive Power and GE for systemic repairs.


"I don't think we expected to get anything with no defects at all," he said. "Our intent is to make sure that we can manage those defects so that, in the end, the people who pay for the locomotives are getting what they paid for."

The MBTA has already solved some problems, such as malfunctioning fuel valves and electrical problems that led to fires.

Other systemic defects include software trouble.

Of the 961 total delays recorded between June and December, about 32 percent were related to train sets with new locomotives. But the MBTA points out that the new machines — even with their defects — are far more reliable than the old locomotives. Marked improvements in December and January, when all 40 new locomotives were available, bear this out.

On average, five groups of older locomotives traveled between 450 and 6,215 miles between mechanical failures in December. But the new locomotives were able to travel much farther, about 18,273 miles, according to the MBTA's data.

Keolis Commuter Services, which runs commuter rail service for the MBTA, reported that December marked its best on-time record of 2015, with 92.4 percent of its trains arriving within five minutes of their scheduled time. That's a marked increase from June, when 87.5 percent of trains ran on time.

The new trains experienced regular problems as they were being put into service, according to the MBTA's daily mechanical failure reports. Throughout the summer and fall, problems included dead batteries, a software glitch, a faulty speedometer, brakes that wouldn't release, and operator error.


But the MBTA says that in December, there were 10 mechanical failures with the new locomotives. That's compared to 18 and 19 failures with two other groups of older locomotives that operated for fewer miles.

The new locomotives were already delayed from service for months: After they were delivered to the MBTA beginning in 2014, the majority were immediately sidelined for repairs. It took until the end of December 2015 for all 40 to be ready for service.

Asked for comment, Clarissa Beyah-Taylor, a spokeswoman for General Electric, directed questions to Motive Power. Tim Wesley, a spokesman for Motive Power, declined to comment.

Joseph A. English, a union representative for commuter rail maintenance workers, said last year that mechanics feared "ongoing reliability issues," but representatives from that union also declined to comment last week.

The MBTA says it is continuing to train Keolis engineers on the new trains, so it can further reduce human error. Keolis Commuter Services employs its workers, but the MBTA is responsible for buying the equipment.

Keolis, which declined to comment, took over the commuter rail service in the summer of 2014, after beating out the longtime operator, the Massachusetts Bay Commuter Railroad Co. The new company came under fire last winter, after record-setting snowstorms hobbled Greater Boston's transportation network and the commuter rail bore the brunt of the delays.


Since then, the company has made several leadership changes.

So far, the repairs for the defects — including the systemwide replacements — will not cost the MBTA, since the locomotives are under warranty.

But if the two locomotives sent to the Pennsylvania facility have problems that are related to Keolis's maintenance, the T could run up a hefty bill.

DePaola said riders should not be worried about the new locomotives.

"The team is doing a great job of managing through and getting this to the point where they exactly meet the specifications we put out," he said.

Russell Goldenberg of the Globe staff contributed to this report. Nicole Dungca can be reached at nicole.dungca@globe.com. Follow her on Twitter @ndungca.