This could be called The Grudge. During my 2012 Northern Ontario trip I didn't quite get to the Oba GFA, but now know how to get there, so the next time should be much easier than the last time. From where the Suburban will be able to get it's about a 5Km hike to the top of the hill where the two Steelox buildings and 100,000 gallon fuel tank should be.
The dotted line below shows what I will need to traverse on foot, and most of the path is actually an old logging road, now likely used by hunters and such. On the far side of the tracks the road is the original Gap Filler Annex access road. As there was a fire tower at the location, I expect the road will still be cleared of scrub, and shouldn't have any obstacles or wash outs. The distance is ~7.5 Kilometres from the green dot (safe parking spot) and the red circle (the Oba Gap Filler Annex). From the reconnaissance I did in 2012, I can tell you, this is the least problematic way to access the site, unless you know some one who can get you a lift with CN freight or VIA Rail and drop you at the Neswabin siding (and if you do - get in touch!).
Don't believe me? Think there's a better way? Here is the Google satellite view AND Bing view; show me a better way that doesn't involve a water crossing. Also, the rail line is extremely active with steep gravel embankments on either side of the track - there's no way to "walk along the track" (legally or illegally)