The line originally ran up Witherspoon Street to Spring Street in Princeton Borough. Passenger service was first cut back to the end of the private right of way then dropped entirely. The Reading then took over the line and operated it to Lawrenceville, primarilly to serve the Lawrenceville School and a cannery at Lawrenceville-Pennington Road. The line was finally abandoned to avoid the cost of crossing I 95.
In response to the gentleman in another thread who suggested using the R-O-W to extend SNJLRT to Princeton Jct.
The right of way is more or less intact as far as The Great Road in Princeton but is obliterated beyond there. There has also been some residental construction on the line in Lawrence. Even if the line was intact (and relaying the track did not go through a superior court Judge's rose garden) it would still be neccesary to connect from Witherspoon Street to the Dinky Station. I won't say it's impossible but if you look at the curves and the grades and the available space on Rt 27 you will see why its very difficult.
You can still walk the row in Lawrenceville. Half of it is a bike path; the other parts north and south of it stretch into wodded areas. On most parts except in the northern end, you can walk on the rail bed, (without the rails) up until the I-95 crossing behind Rider University. I am not sure of the condition of the ROW in the Trenton area though. For most of the line, there are electrical and telephone cables that are still in use today.