Most importantly as to why they wouldn't just drive to Trenton, this group is as a whole, a highly impoverished part of our society, with few owning cars. Among the obstacles to car ownership is the cost of insurance (NJ has the highest insurance rates in the nation). A male driver, under 25, with NO record of drug or alcohol related charges, would pay about $2,600 annually for basic insurance. Anyone convicted of drug possession (I venture an unsubstantiated guess that includes almost all gang members) would have rates and surcharges totaling several thousand dollars higher, which is unaffordable for most unemployed gang-bangers.
Lastly, NJ has very tough restrictions on new drivers; including the hours they can operate a vehicle and the ages of the passengers that can accompany them in the car.
The RiverLine cost $1.10, and runs till about 1:00 AM, which makes it an ideal facilitator.
BTW, a limitation locals have long demanded to lift, now with success: PTC will be installed on the line to enable parallel running of freight and "light rail". *Lunatic*, n. One whose delusions are out of fashion.