Thank you for all the helpful replies. I am thinking of modeling the mid to late 80's,
Late 80s might be rather good, you can make use of 72' Centerbeam while retaining 60' bulkhead flats for packaged lumber. Also, double stack intermodal started in 1984, so you can justify that.
You can use patched IPD & Railbox boxcars, patched reporting marks like this example (although you'd might use a bit less weathering - the boxcars were only 10-15 years old at the time). Apparently MDC (before Horizon brought it) was releasing kits of it's colorful IPD shortline boxcar with patched reporting marks, but I couldn't find any images. The new-style Hi-Cube FBOX types were a number of years into the future.
SD50 production had finished, SD60 was the new standard EMD - of course, SD40-2 were everywhere. For 4 axles, you had GP60s still in production. No new switchers since 1987, alas.
On the GE side, you have Dash 8-40C, and even some 4 axle Dash 8-40B.
Of course, Wide Cabs were somewhat rare in this period on US roads
We menioned the 52' gondolas, the covered hoppers (stay away from the monsters like the 6350 cuft Trinitys, stick w/ smaller hoppers - they were everywhere), tank cars (frameless, shelf couplers on hazmat, no reflective strips), For reefers, you had the White SPFEs and Yellow UPFEs (+ FGE Solid Cold and a few others) - hmm, there apparently were some of the huge Cyro Trans CO2 reefers running around (founded 1986) - so if you can find those Walther's Models you are good to go.
Note I was a bit off, the BethGon (Steel) version was available in the 1980s, it was the Aluminium version which was introduced in the 1990s.
BN, Santa Fe, UP, Southern Pacific (Rio Grande), Illinois Central, Conrail are all independant - CSX was formed by 1986, and NS by 1982. You'll need to look up other railroads yourself to see if they existed or not by the late 1980s. Of course, there was still plenty of rolling stock labeled with fallen flags like Missouri Pacific or Boston & Maine running around.
And TTX was about 2 years into the future, so everything is labeled 'Trailer Train'
As to actual percentages of road names vs. fleet types vs schemes etc...well, I'll leave that up to you JLK.