What you haul determines what kind of cars you need. If you're hauling passengers, you need passenger cars. For granular substances like gravel, coal, concrete, sugar or grain, try hoppers, ore cars and gondolas. Logs use log cars, flats or gondolas. Liquid freights can be shipped in tank cars, or the liquids packed in containers can be hauled by gondola, reefer or boxcar.
Perishables go by reefer.
Smaller dry loads are the province of the boxcar, and anything too big for the boxcar goes by flat or gondola. Whimsey fans can invent their own peculiar cars. Among the few non-cargo trains would be firefighting trains, armored combat trains and maintenance of way. Railways can be old or new. Most are set within a particular era.