As others have said, it's your railroad so you can run what you like. However, we do spend a lot of money and effort to get highly detailed models of specific prototypes - otherwise any old pile of crap would do the job. In that sense, we all have some interest in realistic models, not just the specific cars and locos but in consists and their surroundings as well.
Others have already given the build dates and the general lifespan for locos. I would only add that larger Class I's tend to use newer locos in mainline service than 40 year old derelicts. A GP9 is a front line loco from the 50s-60s-70s but after that, they were relegated to yard duty, branchline duty, rebuilds or just sold to shortlines. Yes, one can find exceptions but when we model we tend toward the normal practice.
On rolling stock, the eras are divided up in several ways not necessarily corresponding to the introduction of loco technology. I will give a very brief summary of the 20th century ...
1) wood cars with truss rods generally prevailed up to about World War I
2) steel hoppers were in general use around 1900 and other steel cars followed so that new construction in steel was the rule by about 1915
3) wooden truss rodded cars were banned from Interchange service in the early 1930s as were billboard reefers
4) double door auto cars came into general use in the 30s
5) brake wheels were moved to car ends in the 1920s and the above roof brake staffs vanished before the Second War
6) post World War II the 40 foot car, especially the general service boxcar, reigned supreme until the late 50s
6) post 1960 there was a rapid evolution in freight cars as they were lengthened and strengthened and made more specialized - the general purpose 40 foot box was on its way to extinction
7) during the 60s wooden reefers and stock cars were withdrawn from service, steel reefers in small numbers were in use but slowly died out - today reefers are very rare with a few notable exceptions (like the Tropicana OJ train)
8) roof walks were removed from house cars by 1970? and the brake wheels were moved lower on the car ends - for safety reasons, brakemen were not permitted on the car roofs
9) during the 1970s all cars carried COTS stencils but technological evolution eliminated these in the 1980s
10) with the replacement of the huge fleet of general service 40 foot boxcars with 50 foot or longer cars came specialization - by the 1970s, grain was shipped in covered hoppers, auto parts in special high cubed cars, automobiles in auto racks and the biggie, general merchandise in trucks or in some cases in TOFC; chemicals especially hazardous chemicals were shipped in a huge array of tank cars
11) by the 1990s this evolution was almost complete to where the preponderance of cars on the rails were covered hoppers and tank cars, followed by coal hoppers, ore jennies, and other special cars - the general service boxcar became a fairly rare beast
I have left out a huge amount of detail and said very little about tank car evolution and did not really mention hoppers or gons. The point is simple though; just as locos have evolved so have freight cars. And while we most often think of our eras in terms of loco technology, freight car technology is just as important in defining the look and feel of a specific timeframe.
Regards ... Doug
