Turning and tunnel constraints
While some of today’s trains do turn at Penn Station, all of the trains must turn somewhere on their branch lines. Given the highly asymmetrical nature of the demand, with almost all passengers looking to travel to the Manhattan central business district (CBD) during the AM peak and departing from the CBD during the PM peak, station pairing for revenue-to-revenue through-running would generally be relatively close to Manhattan. It is not practical to schedule trains from the end of one LIRR branch to the end of a NJ TRANSIT branch (such as Babylon, NY to Trenton, NJ) because the passenger demand doesn’t exist to justify the crew expense and the costs of purchasing and operating the equipment. The most effective utilization of through-running would be for relatively short runs making stops within the metropolitan core and then turning back. For example, a service running from Hempstead, NY to Secaucus, NJ would have higher demand at the terminals than a service from Trenton, NJ to Babylon, NY.
Once these short runs beyond Penn Station are complete, the trains will either need layover space at the destination, or to turn back from the destination preempting trains from further away despite their likelihood of having fewer passengers. For example a train traveling from Babylon, NY through Penn Station and on to Secaucus, NJ in the AM peak would have to layover in Secaucus or turn around at Secaucus and then continue back through Penn Station and on to travel in the reverse peak direction east, back to Babylon. To layover, that train would need to have a railyard in Secaucus where it could be stored until making its return trip to New York and Babylon. Because regional demand is not balanced – many more travelers want to access Penn Station in the morning, and many more want to leave in the evening – it would not be beneficial for the railroad to turn that train at its terminus and immediately run it back through the core of the region. Doing so would compromise peak direction capacity.
During the morning peak, when demand to enter the core is highest, this train turning at Secaucus and traveling East would add to the queue of trains traveling into Penn Station during the AM peak, competing for limited tunnel space with trains traveling from further out. In this scenario, we would expect that the train turning at Secaucus will have relatively few riders compared to trains traveling from further away, because it will have served far fewer stations. In the PM peak, the train turning at Secaucus will be traveling against the regional demand, once again reducing overall capacity.
Another constraint is that many parts of the system rely on sections of single tracks while others rely on multiple tracks with unequal travel.
For example, in the Kearney-Milburn section: there are three tracks, two tracks devoted to the peak direction, and one track devoted to the reverse-peak trains. The single reverse-peak track limits the ability of this line to receive LIRR trains and provide both local and express service to the Morris & Essex Line.
Many branch lines throughout the region have portions of single track. New tracks would have to be laid throughout for a consistent through-running operation.
Branch combinations and connectivity
While a train enters and exits Penn Station on average every two and a quarter minutes, these trains are convergent and not substitutable. Unlike the NYC subway, the Paris regional rail (RER), or the Elizabeth line in London, where trains follow each other along the same paths, the intensity of service at Penn Station is the sum of one to four trains each hour on branch lines that all converge at the tunnels leading to Penn Station. With 10 branch lines on either side of Penn Station there are 100 branch-to-branch combinations. With through-running, the odds that the branch you started on is scheduled to continue on your destination branch are very low. Hence the connectivity upgrade would also be very low. In other words, even in a revenue-to-revenue through-running service, if you are a passenger traveling from Long Island on the Hempstead branch and going to a New Jersey destination on the Gladstone branch, you are likely going to need to transfer at some point during your trip, because the train you boarded on the Hempstead branch could be connecting to the NEC, or Morris & Essex or some other branch at the time you wish to travel.