 The extension replaced the Orange Line portion of the Canal Street incline, where the old El came to the surface north of Haymarket, with a tunnel under Canal Street to a new underground station at North Station, replacing the elevated station.The tunnel continues north under the Charles River to Charlestown, with an aboveground station at Community College that roughly replaced the former City Square and Thompson Square stations.The extension runs on the surface under the Interstate 93 elevated viaduct - constructed at the same time - and crosses the Haverhill Line and Newburyport/Rockport Line on a flyover, touching down at Sullivan Square (which replaced a nearby El station of the same name).