There was an earlier thread on this topic, I forget where. As I recall, there is a big elevation difference between Stewart and the Graham Line. It may be a short distance as the crow flies, but the RR route would be much longer.
The TZ project seems stalled by local politics, NIMBYS and lack of money. Also, every time it is talked about, the cost estimate rises spectacularly.
I think the best we can do for Stewart, for at least the next generation or so, is improve, maybe double track the Graham line, improve Bergen line, run faster service to SEC and HOB, run a shuttle bus route from Stewart to the Graham Line. If demand warranted, a new station could be built.