Some of the different categories are:
1. Type of material being transported - coal cars, lumber, or mixed freight.
2.Type of engines being used - steam era, early or late, diesel early or late
3. Period - which will determine what detailing to buy. If you model the 50s, you will need to only buy 1950s automobiles and earlier, structures suitable to that time period
4. Area being modeled - modeling a country area, or city