Transit Police arrested a man accused of attempting to expose himself and of assaulting an off-duty MBTA inspector yesterday on a Green Line train, according to authorities. Darrell Oliver, 56, was charged with lewd, wanton, and lascivious conduct and simple assault and battery, a Transit Police report said. Oliver allegedly committed a lewd act and unzipped his pants while on the train, prompting a passenger to call police. When the train pulled into Park Street Station, officers saw him allegedly punching an off-duty T inspector. According to the report, Oliver also used racial slurs and threatened to beat up officers when they arrested him.