I say go ahead with it. There are going to be NIMBY's wherever they try to go. Thornburg to me seems to be the perfect place for it. Last time I was through there(Last summer) the only thing there in that location was trees and cow pasture, and on the other side of 95 there is a McD's and a hotel that has been in the construction phase for about 5 years now.
Having grown up in the area, I have heard this kind of opposition before. I lived in Stafford before living in Spotsy. Along 95 in North Stafford, where the Airport is at now (of which I hear people complain quite a bit about) they proposed building Redskins Stadium there on that bit of land. Easy exit on/off 95 were to be built by the stadium too. The NIMBY's screamed and hollered about it complaining that the noise would be too great (really...how many football games are there a year?) Now they play in MD and Stafford got an airport that operates 24/7/365.
Where it in now in Manassas it is really out of place. Bring it.
Hell yeah! More tracks please! More places to go. I talked to my friend out in California, and he said they have pretty much shut all the tracks down. And, the tracks that are left are 1/8th only....what a crap deal. I don't want it to be like that over here.