Got arrested on the NYC subway for having a pocket knife?

I was transfering trains, tha L to the A and plain clothes officers swarmed me and arrested me for a little 3in pocket knife i had, the clip was showing on my pant pocket. Im a mechanic so i use it for work and i was coming home from work. i never seen a sign or any rules saying that was illegal or i would not of done it. I just moved to NYC 3 weeks ago and just started riding the subway. anyways i spent 5 hours in a jail cell with a bunch of other people, then they let me go with a court summons. Anyone know the legalities of this? I have a record for Duis in California and im on probation for that, but nothing else. Im not a convicted felon. I really hope they drop this charge, i dont need more probation or another misdameanor. Thanks