I designed this railroad using Atlas code 100 track with number six switches for someone who has little space but wants to try everything. It has space for rolling stock and a locomotive. It has space for landscaping and structures. And it has space for operations.
It has three railroads. One railroad runs across the track at the front of the shelf. Another other runs across the right corner. The third is the industry's switcher. You can emphasize that these are different railroads by using different color ballasts on them. A tower guards the crossing between the two main line railroads. We never actually see any trains on either of the main lines; we only see the cars they set out on the interchange track.
For operating purposes, the industry bought trackage rights on the east/west main so it could pick up and drop off cars at the interchange. Thus the industry's work consists of picking up cars from the industry tracks, moving them to the interchange track where they are swapped for arriving cars, then setting out the newly arrived cars. This process is repeated each time the main line trains bring in more cars.
The industry can be any big industry you'd like. The chemicals and fuel tracks on my labels imply a paper mill, but it could be whatever large industry appeals to you. For instance, shipping tallow instead of receiving chemicals could make this a meat packing plant. (For more ideas on big industries in small spaces you might want to read my article in Model Railroad Planning 1999.)
I hope you enjoy building and operating this model railroad. Doing so is the best way I know to develop skills.