I have two reasons that nobody should be working particularly hard to send Long Island trains to Boston:
BACKTRACKING
1a) it involves too much backtracking (a c shaped path); going west to go east
1b) There's no place to do the c anyway
1c) No train should ever plan to stop short of, or turn away from, NYP (except for tunnel closure emergencies)
NORTH-SOUTH IMBALANCE OF DEMAND
2a) the Southside (WAS/HAR/PHL) has lots of trains that currently terminate at NYP could be extended onto Long Island.
2b) the Northside has low demand density and restrictive CT slots, and therefore no extra frequencies terminating at NYP that you might extend to L.I.
2c) If there ever were additional trains added Northside it would always be a better idea connect to Philadelphia and Washington rather than turn them onto Long Island
BACKTRACKING
1a) it involves too much backtracking (a c shaped path); going west to go east
1b) There's no place to do the c anyway
1c) No train should ever plan to stop short of, or turn away from, NYP (except for tunnel closure emergencies)
NORTH-SOUTH IMBALANCE OF DEMAND
2a) the Southside (WAS/HAR/PHL) has lots of trains that currently terminate at NYP could be extended onto Long Island.
2b) the Northside has low demand density and restrictive CT slots, and therefore no extra frequencies terminating at NYP that you might extend to L.I.
2c) If there ever were additional trains added Northside it would always be a better idea connect to Philadelphia and Washington rather than turn them onto Long Island
"Trying to solve congestion by making roadways wider is like trying to solve obesity by buying bigger pants."--Charles Marohn