I think there were 3 groups of imports over the years. The first was by NJ Custom Brass - they did the S1, S2, S3, and S4 for sure.
I think Overland Models (Green Box?) imported at least some of the models if not the whole series. I have seen OMI S4s for example.
The last group of imports that I remember was "The CarWorks". They also did all 4 models I think. I have seen Carworks models with both Blunt and ARA trucks. Carworks also import Baldwin switchers and FM Roadswitchers.
There is a website http://www.brasstrains.com/brassguide that might list most of the models. this site catalogues models as they get information, so it is not necessarily complete. I think they also buy and sell models.