The drive between Boston and Cape Cod can be 60-plus miles of nightmare, depending on the time of day and road conditions, as surprised commuters discovered on Mother’s Day, when construction allowed only limited access to the Sagamore Bridge. That work should wind down by the end of the month, but seasonal slowdowns await. So it’s disappointing that a plan to extend weekend service on the MBTA’s Middleboro commuter rail line to Hyannis this summer has been shelved for now.