- When you have a car, every trip is a marginal cost. You're paying for the vehicle, maintenance, and insurance anyway - everything but the gas and possibly tolls. When you don't have a car, you pay the full cost every time to either rent a vehicle (plus gas and tolls) or take a train. Therefore, each trip feels more expensive.
- Very few true hikes can be reached via the NYC subway/bus system, but these trips are free if you already have an unlimited monthly pass.
- If you and four friends decide to take a car to a hike, the cost per person is divided by four (plus the negligible expense of extra fuel because of more weight). If you and four friends decide to take the train, there are no cost savings.
There are other important factors besides cost; some hikes just can't be reached by public transit. Also, taking the train requires more planning, in many cases. Some stops are weekend only, with just one or two stops a day. You don't want to miss that train.
The net result is that it takes more forethought and planning to hike out of the New York city area without a car, and a couple of too many train hikes could quickly wreck your budget - an off-peak one-way ticket can cost up to $18.25 on the LIRR, $16.25 on Metro North, and over $20 on NJ Transit.