Convention to hit North Shore rail the hardest

Commuter rail riders on the Newburyport-Rockport line will face a much tougher morning during the Democratic National Convention than their counterparts on the Haverhill, Lowell, or Fitchburg lines, MBTA officials say.

During the week of July 26, riders coming in on the Fitchburg line will transfer to the Red Line at Porter Square in Cambridge. Haverhill line passengers will transfer in Woburn to shuttle buses that will have their own dedicated lane on Interstate 93 into Boston. And those on the Lowell line will transfer to the Orange Line at Oak Grove in Malden.

But the estimated 8,700 weekday riders on the Newburyport-Rockport line -- the most heavily used of the four commuter lines emanating from North Station -- must transfer at Lynn to shuttle buses that will have to fight traffic on Route 1A on their way to South Station.

There will be no dedicated lanes for those buses and the riders can't be transferred to the Blue Line at Wonderland because that part of the subway system can't handle so many people, said Michael Mulhern, general manager of the Massachusetts Bay Transportation Authority.

''We're dealing with the heaviest commuter rail line matching up with the smallest rapid transit line," he said.

The Blue Line operates with four-car trains. There is a plan to extend that to six cars, but that requires extending the platforms at all Blue Line stations, a project that is underway. More than 90 new Blue Line cars are also on order, but won't be here by this summer, Mulhern said.

Riders on the Newburyport-Rockport line also have the option of continuing on to Chelsea and using existing MBTA bus service into Boston from there. But those buses will have no special lanes, either, and will take riders to South Station instead of their usual destination of Haymarket.

For the return trip, riders should take the Blue Line to Wonderland, where they will be put on shuttle buses to Lynn to catch trains taking them to points north, Mulhern said. The shuttle bus from Lynn to South Station will only operate in the mornings, before 10 a.m., he said.

The T will closely monitor the progress of the shuttle buses and if they get bogged down in traffic on Route 1A, riders might be diverted to the Blue Line after all, Mulhern said.

''We're going to focus on this line more than any other," he said. ''The potential for inconvenience is much worse. It's a tough corridor."

Zachary Greene, who normally boards the Newburyport-Rockport line at Swampscott to get to his job in Back Bay, said he is staying in his in-laws' vacant apartment downtown that week.

''I'm lucky," he said. ''It makes sense they can't dump all those people from commuter rail onto the Blue Line. But with all the traffic already on 1A, and they are going to add a stream of buses to that? I know it's not going to work."

Anthony Flint can be reached at flint@globe.com.