Building your model railroad is all the more fun when there is a sense of purpose behind it. After all, railroads do not just spring up as "general traffic" operations. Even the big Class 1s started out as short lines with a specific purpose in mind.
Fuel is another thought. Most factories have to power their equipment. While some may use the local electrical system for power, others may have steam or gas-powered machinery. There are even companies who might very well have their own little power plant producing their electricity. Your railroad will have to bring them coal or oil.
Here is where those fun O and S gauge operating accessories come in. Operating milk cars can work with the dairies. Operating cattle cars would supply the meat packers. Barrel loaders could load almost anything shipped in a drum: lubricants, industrial materials, fish, clams, and much more. (The clams and shellfish are usually packed in water with ice). Log dumps can drop their loads at furniture factories. Coal dumpers can supply factories and power plants.
Passenger roads have different requirements. Commuter and passenger trains also carried cars for express (less than carload) service and milk runs. Those with a smaller number of passengers might use combines rather than coaches and baggage cars. Also think of seasonal traffic. Resort and vacation spots might have few riders between October and May. From the end of May to the middle of September, they will be filled with vacationers and "day trippers" out for some fun in the sun.
These may seem like fine points, but they are drawn for real life. Certainly, incorporating some of them into your railroad will add to its realism and your enjoyment of it.
The needs of businesses and communities changed from one era to the next. You can choose to factor this into your railroad, or not. Some see it as a fine point and others regard it as necessary for scale railroading. A mentioned earlier, coal was a popular fuel from after the middle of the 19th Century until the early 1950s. From the 1930s to the 1990s, oil heat was popular. A railroad strictly set in the 1920s would be hauling coal to its communities. One set for the 1960s would have tank cars for heating oil. Wood-burning locomotives were still running in the 1870s but by the 1880s coal-burners were the order of the day. If your railroad adheres firmly to a specific time, then it pays to make your consists fit the era.