NJ Transit out of NY Penn Station to New Brunswick, there are no bike permits, it is not an issue. Return from Trenton, on the same line out, but 3 stops further than New Brunswick.
4.6 miles on one road from New Brunswick to Six Mile Resevoir Site. It is all single track; it flows. No rocks, just roots, occasional logs and narrow bridges for stream crossings. Lots of poison ivy, stay in the center of the track and watch where you put your foot down. Don't miss the left turn to stay on the red trail, near the beginning. If you do, you will do as I did and notice that the sun has swung around, and parts of the trail look like you have been there before. The red and blue trails are well worn. If you notice things getting thin, you missed the trail. By the time you have come to the end of the trails on Canal Rd you will have done 7.5 miles of twisting single track. You will be ready for the towpaths. But be cognizant that these trails have 2 way traffic. Chances are that you will meet others coming head on. Be prepared to stop and work out who goes by first. Only a highly skilled rider should attempt Six Mile Run on a cyclocross bike. It appears to be suitable for a non-suspended mtn bike, although front suspension for riding over roots will make the ride more comfortable.
The East Coast Greenway coincides with the towpath as you near Trenton. If you stay on the towpath beyond the left turn onto North Montgomery, in a block or 2 on your right you will see a statue of George Washington high on a pedestal. The neighborhood is a bit run down but one day will surely be restored back to its colonial glory.
If you have a lot of time on your hands you could stop in Princeton for lunch, or continue on the towpath to Calhoun St, take a left and cross the bridge over the Delaware, pick up the towpath along the Delaware Canal going north, go to Washington's Crossing, cross that bridge and return back to Trenton on the D & R Canal towpath. There will be a few times that you will have to cross a bridge to contiune on the towpath as it will be on the other side of the canal. The path will be on one side or the other so don't despair. At Mulberry St, unmarked, just on the outskirts of Trenton, the path leaves the canal as you go south to pick it up in one block, just beyond the highway overpass. It now becomes paved asphalt. Trenton has social issues but I found users along sections in the inner city, which gave me a sense that the community deems it safe.
You could do the whole thing on towpaths our of New Brunswick and avoid mtn biking and 6 Mile Run altogether using this route: https://nycc.org/cue-sheets/central-jersey/79745. If so, a cyclocross bike or wider tires on your road bike will do on the towpaths.
Once you leave Rt 27, there are no services directly on the route, but you certainly can cycle into Princeton.
This route was originated by John Z.