The man in the suit, after spending nearly 45 minutes inside the engineer’s cab of a morning-rush hour Long Island Rail Road train, emerged and said jokingly to a fellow rider, “How’d I do?” said the MTA Police’s key witness in the ongoing investigation of charges that a passenger operated the train.

The witness, who was on the rush-hour train Friday morning and spoke under the condition of anonymity, nervously recalled what he saw on July 2, which sparked him to call police shortly after stepping off the train at the Hunterspoint Avenue station in Long Island City.

“I thought about it . . . and I just said, ‘You know what? If something happened and there was some kind of accident and the railroad figured out that the engineer was away from the controls and I knew it . . . ‘ ” the man said, trailing off. “He was putting my life at risk and everyone else’s on this train.”