Your motive power makes it much tougher. The GG1s ran in the NE Corridor and out to Harrisburg. The turbine ran way out west between Chicago and crestline or there abouts most of the time. At least the photos I see of the turbine it is out there.
My choice would be to model a branch line such as the Elmira Branch or the south end of the Monongehela branch that are single track with passing sidings. The Mon was double track at one time. There is also a Southwest Branch (which I model) which ran from Greensburg to Uniontown with a big yard at Youngwood. There were lots of branches off of this serving various coal mines and some small towns.
On a 4x8 it is hard to do even double track, so go with the single track. Another idea is to run a stub end passenger station off the loop on one side at a diagonal across the layout from one end to the other. Perhaps two tracks. then have the passing siding on the other side of the loop to hold the turbine with a freight and a stub siding or 2. Put a kill switch for both the passing track and the PSG station. You can park the GG1 in the passenger station and turn it off and then turn on the freight and run it. Park the freight in the passing siding and tutn off and then turn the GG! on run it..
