Ron, that's my little secret.
Seriously, it is a great idea and can save a lot of time when leaving the pier. The more ships in port, and the later you disembark, the more of a nightmare the taxi line becomes. Actually, it turns into a disorganized mess. Do not, however, accept a ride from a driver who solicits your business inside the terminal. Instead, walk toward 11th Avenue, and you will find a taxi very quickly. At least we did on several occasions.
As for the train -- superb idea. The parking rate at the NY Passenger Ship Terminal is currently $24 per day. Depending upon the number in your party, you may save a few bucks by taking the train.