By shifting from a commuter-focused model to a "regional rail" model, the MBTA's Greenbush, Kingston/Plymouth and Middleborough/Lakeville lines could run trains every 30 minutes, shave 15 minutes off trips into Boston and make public transit a more realistic option for many, a new report suggests.

TransitMatters also said an investment of $630 million could electrify the Old Colony trunk line and its three branches to allow for speeds of up to 100 mph, double-track congested sections like a bottleneck between Dorchester and Braintree that causes scheduling issues, and make adjustments to track alignments at Quincy Center.

With those changes, the lines that have some of the lowest ridership and frequency levels could instead "provide frequent, affordable connections from the South Shore communities to Boston's job market and to each other, particularly Brockton and Plymouth," the nonprofit advocacy organization that regularly seeks improvements to the transportation system said in its report.

"Brockton, Plymouth, Randolph, and other South Shore communities have dealt with slow, infrequent rail service for too long," Ethan Finlan, TransitMatters' regional rail campaign director, said. "The steps in this report will make the South Shore's rail transit much more accessible and useful."

The report identifies Brockton as "the most important location on the Old Colony system outside Boston and the trunk" and concludes that "it's crucial to build a system that helps Brockton grow as a regional center" because of its population, the concentration of jobs, and development opportunities.

Right now, TransitMatters said, the most common way of commuting for the 9,000 Brockton residents who work in Boston or Cambridge is to take either the Brockton Area Transit #12 bus or MBTA's Route #240 bus to Ashmont, then transfer to the Red Line to get downtown or to Cambridge. Getting to South Station takes just more than one hour.

Taking the commuter rail is faster, but also more expensive. The hour-plus trip on the bus and Red Line costs $4.65 one-way or $155 monthly while the commuter rail costs $8.75 one-way to South Station or $281 monthly, the report said.

If the recommendations of the TransitMatters report were to be adopted, the group said, Brockton would have a direct train to South Station at least every half hour and, combined with a fare integration policy, could cost about the same as the fares on the MBTA's subway system. The trip to South Station would take about 25 minutes.

"And it's not just for commuters," TransitMatters said. "All-day service would get tourists to attractions in Plymouth, students to and from Bridgewater State University, and workers and professionals between Brockton, Braintree, Quincy and Boston at off-peak hours."

If the proposed changes become reality, a trip from South Station to the end of the Middleborough/Lakeville Line would take 42 minutes instead of the current 59 minutes, and the line would be extended to Hyannis, an end-to-end trip that would take 1 hour and 19 minutes. Going end-to-end on the Plymouth Line (which would be extended to a new Plymouth Center station) would take 45 minutes compared to the current 57 minutes to North Plymouth. And a trip from South Station to the end of the Greenbush Line would take 36 minutes instead of the current 59 minutes, the report said.

The $630 million price tag included in the TransitMatters report includes $290 million for electrification of the rail lines, $260 million to build or renovate platforms and stations, and $80 million to install a secondary track at key locations.

"There is great potential for ridership on the Old Colony Lines, thanks to the already-existing development in Brockton and Plymouth and student travel to Bridgewater State. However, the Commonwealth must be ready to invest significant sums of money into infrastructure upgrades," the organization wrote in its report. "Without those, ridership will remain weak."

Top state transit officials have already acknowledged that the MBTA's rail system is moving away from primarily serving the morning and evening commuter rushes and towards more regular service throughout the day, influenced in part by the changes in commuter patterns spurred by the pandemic.

"How about we just abandon the word 'commuter' and just call it regional rail? That's where we're headed," Fiscal and Management Control Board Chair Joseph Aiello said in February after T officials laid out changes to the spring commuter rail schedule. "This is really the first major move, operationally, to that. That's likely the future. We may see a peak return to it, but what you've laid out here for this next step is likely to reflect the kind of service that people are going to need, that people are going to rely upon."