To "join" Amtrak, railroads had to buy in, either with cash or equipment (I imagine it was mostly the latter). In return, they received shares of Amtrak stock, which are of course worthless. Rock Island didn't buy in because of its state of extreme poverty. Rio Grande, Southern, and the Georgia Railroad stayed out of Amtrak for their own reasons. Thus, all of the above continued to operate their own passenger trains for several more years. The South Shore was considered an inter-city carrier and stayed out, although it was and is really a commuter operation. Kalmbach's book, "Journey to Amtrak" is a good source of information on the whole process.