Light rail up the W&A, then the Hook 'n' Eye to Canton is pretty much unfeasible ... too curvy, too much freight, and two railroads who likely have no inclination whatsoever to cooperate ...
Freight currently operates max speed of 45mph on the W&A up to Marietta, and 10mph on the GNRR's trackage ... and even if you rebuilt the GNRR track to perfect standards, I doubt you could get much over 20-25, and you'd need seatbelts for that.
Their best bet is new-build, and that's basically the lesser of two evils :
1) try and acquire ROW (very expensive; it's pretty much ALL developed land)
2) build down the middle of the interstate and drive 1/4th of the city nuts with the traffic jams that would create ...
Frankly, if they're gonna build down the middle of the interstate, uncap MARTA's tunnel, and just extend the NW line that was part of the original planning. Shoulda happened years back as part of the pre-olympics planning IMHO ...
'course for ANY of this to happen, the state government has to manage to actually put together some kind of realistic rail goal/plan, something that's sorely lacking (and less likely).
- litz