Gangs of wilding teens terrorized straphangers this week in a violent spate of daytime robberies and assaults on Manhattan subways -- another indicator the city could be sinking back to the bad old days that once gripped the Big Apple with fear. Eight teens were arrested in the spree, which included a stabbing and four cellphone robberies on the B and F trains in Midtown Tuesday. The rash of crimes came two days after an alarmingly violent prior week, when subway felonies jumped 131 percent, from 22 to 51, compared to the same period last year.