Complex sentence: South Rail Link has been proposed to unite the two halves of the commuter rail system; but, because of the high cost, Massachusetts has withdrawn its sponsorship of the proposal, in communications with the United States Department of Transportation.
Split into simpler sentences: South Rail Link has been proposed to unite the two halves of the commuter rail system. However because of high cost, Massachusetts withdrew its sponsorship of the proposal, in communications with the United States Department of Transportation.