There are so many bottlenecks in the WAS-BOS corridor that limit the number of potential trains. Whether it's creaky old bridges with speed limitations or those limited to X trains per day per contract with the Coast Guard, or ancient two-track tunnels and bridges, there's no way to add even one more train without impacting the reliability of others. Throw in the abundance of commuter trains from the agencies the_traveler indicated above, and in 10 words or less...the NEC is at or above capacity already.
At least, during the weekday, Amtrak has a train each way NYP-WAS about every 30 minutes. As for adding more stops along the way, that's the realm of the commuter agencies, not Amtrak. There must be enough Amtrak passengers, however, for a few trains to stop at Metropark, Princeton Junction, and a couple of other small stations along the way.
New equipment would be nice. But until more of the bottlenecks are removed, there can't be any increase in train frequencies.