Casey,
What I have observed is that loads coming off of ST and going west on SLR are destined for CN customers, mostly in the Great Lakes and Upper Midwest states (Michigan, Indiana, Wisconsin). I don't see loads going to CSX, CP, or NS destinations. As you noted, those probably are interchanged either at Northern Maine Junction (CP) or Ayer (NS). I guess CSX is no longer an interchange, since they now own ST.
Greg,
I have been following the few loaded AIMX scrap gondolas that I have seen moving west in the last month or two, and those are also going to CN destinations (e.g. Port of Quebec, Montreal, Wisconsin). I wonder if the predominant flow of empty AIMX gondolas moving east on the SLR are moving from Montreal Est, where American Iron & Metal has a large facility, for loading at the several yards in Maine.
Best,
Fritz