Depends on many things
How good the LOD's are on the railcars, how many trains are running on the rest of the route.  JR have a nice little script that deactivates the physics on uncoupled rail cars, but 600 to 800 cars in a top line i7 is doable,  I've done 2,600 moving rail cars at one time not all in view though, but that's with very sparse scenery on the route.  Today I can't get that many as I run few complicated scripts that seem to hog a bit more of the PC's resources, T2 should rectify this though