The two-car train in the photo is the Princeton Dinky, a train line that runs about 2.7 or 2.8 miles between downtown Princeton, New Jersey (and the Princeton University campus) and the main line railroad at Princeton Junction, for a travel time of about five minutes. The name of the very small train and very short line is now officially, I think, the New Jersey Transit Princeton Shuttle, but the old name “Dinky” has struck with it long after the little steam engines that gave it that name have been replaced by electric service. There are two reasons to talk about the Dinky, one from the past and one for the future.
The obvious question is why the main line misses a reasonably important stop like Princeton, making the Dinky a necessity. The answer is in the (as always) convoluted history of US railroads. The state of New Jersey is shaped like a stylized S with the large cities of New York and Philadelphia about 90 miles apart across the middle neck of the S. As the two largest cities in the country for most of the period from 1790 to 1880 (Philadelphia temporarily dropped down the list between 1830 and 1850), travel between them was always a hot topic. The Camden and Amboy Railroad was one of the earliest passenger roads in the US, running from Camden, New Jersey (directly across the Delaware River from Philadelphia) to South Amboy, New Jersey (at the far southwestern corner of Lower New York Bay). Ferries were needed at both ends to connect to the big cities, but the route was an almost straight line. A few years later a parallel railroad, the New Jersey Rail Road and Transportation Company, was built running from Jersey City (directly across the Hudson River from New York) to Trenton, New Jersey. The two railroad eventually merged and were eventually merged into the much larger Pennsylvania Railroad, giving the Pennsylvania direct access to New York. (The whole mess is described here if you really want to know.) The obvious question of why one or both railroads hadn’t been aligned a bit differently to run, say, straight from Jersey City to Camden has a reasonable answer: to avoid crossing several small rivers at shallow angles. The NJRR came close to Princeton but missed it in order to run parallel to Stony Brook and the Delaware & Raritan Canal. Eventually a branch was built off the main line to cross the stream and canal to go to Princeton, and that trackage is the origin of the Dinky.
As for the future, not every mass transit line needs to be perfect. It would be better for people going to Princeton if a one-seat trip were possible, but where would it go? New York? Philadelphia? Somewhere else? As it is, the one-stop shuttle takes you to a station on a very busy main line with regular service to a lot of destinations to the north and south. It seems to me that an easy way to serve destinations near but not on existing main lines is this kind of shuttle, whether heavy rail like the Dinky, or light rail, or even busses timed to meet the trains.