STRAIGHT SHOT Metcalf & Eddy’s Martin and MWRA’s McBride (right) helped guide complicated tunneling to ease overstressed facilities. (Photo by William J. Angelo for ENR) Six south shore Boston communities are getting a new $200-million sewage pumping and transport system to help reduce chronic surcharge and backup problems. The work involves utilizing three different tunneling techniques to cross a river, peninsula and bay. Work on the Braintree-Weymouth relief facilities project started in 1999 and will wrap up by year’s end. The effort includes constructing a 45-million-gallon-per-day intermediate pump station; 2.7-mile-long, 42-in.-dia hard-rock tunnel under Boston Harbor; 60-in.-dia relief interceptor; and