I don’t know of any lists that exist. I think you would need to do a little research and use some common sense. Also take into account the time period you are modeling. Different things shipped by rail 60 or 80 years ago than today. Geography will also be a factor. Farm country produces animals, milk, corn, grain, fruit, etc. Forests produce logs. Saw mills take in logs and produce lumber. Furniture manufacturers take in lumber and produce furniture. Bakeries take in flour and produce baked goods. Slaughter houses take in animals and produce meat. Mines dig out rock and produce coal or ores. Steel mills take in ore or scrap metal and produce finished sheet steel in rolls, I beams, rebar, pipes, etc. Paper mills take in wood pulp and produce rolls of paper. Printing companies take in paper rolls and produce books, news papers, catalogs, etc. Years ago, general stores received their variety of goods by rail cars and sold to the general public. I’m sure you can think of lots of other examples.
A model train switching layout will provide years of interesting
and challenging operations. As you realize, businesses must
have incoming freight, and then outgoing products. Some, however,
only receive products...I think of food distributors or petroleum dealers.
You would get the most action from small companies...they take up
less layout space, yet use a lot of freight shipments. Many of the
businesses in your town used LCL (Less than Carload) freight stations
for their shipping. The railroad had a special building with car loading
and also truck loading docks.
I used a car card system to make my switching sessions more
enjoyable. I created a card for each loco and car. The card
had type (Box, Reefer, Gondola etc.) Road name and
car number. It also had short descriptives: colors etc.
To make up a switching session, I used 'Post its' that
listed where that car is and what is to be done with it (build a train
or shunt to a freight user). You use these to set up your
operation then follow their orders just as does the real
railroads. If you have family or guests you can make up
operations for them.