Trying to find other sources of income to ease the financial strain on riders, the MBTA plans to install digital advertising displays at some of its busiest stations later this year, a move officials believe could generate $1 million annually atop conventional billboard income. Riders who cringe at the memory of T-Radio - the inescapable mix of music and ads piped into stations but quickly abandoned amid withering criticism in 2007 - may be relieved to learn these ads will be silent, the state’s top transportation official said.