Trackside industries make your railroad come alive, and can add to both viewing and operating interest.
What I want to talk about are those industries that are small and can be located on one siding.  One example is that of a fuel oil dealer.  Just about every town in the US had at least one fuel oil dealer that had its facilities located along a siding near town.  These dealers handled fuel oil, gasoline, diesel fuel, kerosene, lubricating oil and grease, along with other petroleum products. Some even had gasoline pumps so that customers could fuel up their cars or trucks.
    These facilities were usually located at trackside so they could receive products by tank car.  They typically had several storage tanks for holding various grades of fuel oil and gasoline.  There was usually a structure for unloading and storing drums of oil and packaged products, and a loading rack for filling delivery trucks.  To unload fuel from tank cars, a hose was needed to connect with either the car's bottom outlet or a fitting on top of the car.  The fuel was then pumped into the storage tanks.
    To build this facility you will need storage tanks which can be either horizontal or vertical depending upon your space, a pump house, a storage shed for bulk products, an office, pipes for tank car unloading, and a rack for hoses to load your trucks.  You can also position some smaller tanks around the area for lesser petroleum products. Many of these structures can be purchased from Pola or Piko, but to get a more distinctive look, scratch building seems to be the key.  Various sized PVC from Lowes or Home Depot can be used to make oil storage tanks with styrene cut for the ends, and your office, pump house, and warehouse could be scratch built from wood or styrene.  Grant Lines makes some great door and window castings.  As far a rail facilities go, it only takes one small siding to service this type of business.  Just make sure that your siding is at least one car length longer than the number of cars you wish to spot on this siding.
    Other small trackside industries could include a single grain elevator which would serve a farm community, a small coal yard similar to ones that served towns when coal was the main means of heating, a small stockyard with one or two loading pens, and any sort of small factory, or assembly plant, a lumber yard or saw mill.  You do not have to have some huge factory complex to provide for believable car spotting and pickup from your sidings.  Look at books and magazines, many businesses were served by rail in small towns across the country.  In fact, a large platform located on a siding could serve many businesses in one town. Where I grew up we had a lumber yard, coal and oil facility, and a small warehouse served by one long siding off the main line.  You do not need a major industry to justify freight traffic and switching moves.  These smaller industries can provide a great deal of interest on your layout, along with a reason for operations along your main line.
    Kalmbach has a book out called "Industries Along the Tracks" which is a great reference. It is loaded with drawings and pictures of many trackside businesses that you can model.