To my understanding, the AC line has it's own dedicated track in 30th street station
Where did you glean this "understanding" from? The trains run on the Northeast Corridor from Frankford Junction into 30th Street; it shares the Amtrak platforms on the lower level. No dedicated tracks whatsoever; otherwise how could Amtrak have used the line a decade ago?
No one has yet answered what limits the speed of a light rail
Construction, gearing, voltage (if electrified). They would not be comfortable at speeds above 60 mph. They are not designed for long-haul travel.
What is the reason that a light rail can share track with traffic on the street. Is it just weight?
Due to short car length and tight turning radii, such operation is facilitated. Even such operation is not 100 percent desirable, since the vehicles must obey traffic signals, operate at the speed limits of traffic, even get caught in
traffic. Certainly after traveling 62 miles, nobody's going to put up with stop-go operation at the terminus. And since the AC line currently hosts full
four-to-five-car trains (each car 85 feet long, with 100 seats or so), a "light rail" would be a degradation of service.