If you download the Mesaba Electric, here's a link to some additional activity files that will enable you to explore two of the other railroads on that route: http://206.103.49.193/msts/mesabadd.zip
Each of these is a separate railroad that has interchanges. They are the Great Northern and the Duluth, Missabe & Northern. There is also the Duluth, Winnipeg & Pacific, but you'll have to find it yourself.
Dave