Inexperience with TANE seems to be the biggest problem. Strangely, does-cast-shadows doesn't seem to do anything, and there aren't any builtin trees on the route I'm trying to run, and not very many (unique) tree assets. ??
I'm trying to run Rodgers Pass 1888 and there's a billion and a half trees, but it wasn't the trees exactly. Somewhat surprisingly, it seems the biggest graphics demand wasn't the
number of trees, or tree shadows, but rather that there were so many
overlapping shadows. (Many areas could easily have 4 or more shadows cast on the same spot. I had noted that shadows on high, 4x antialiasing and small shadow texture size was actually better than shadows low, 8x antialiasing and shadow texture size large. Normally shadow texture size isn't supposed to be that important. Using 2x antialiasing was a massive improvement, which I didn't expect.
Second, I didn't know that frame rates greatly improve after the first few minutes - I didn't check a setting that long. Or the "load session, quit game, play session again" trick.

Doing that, it turns out I can set shadows on high, and with 2x antialiasing increase tree draw distance to normal and even increase the draw distance from 2500 to 3500 meters, and have it work perfectly fine running a train at 15 mph. Until I stalled out a train that should be heavy on a 4.5% grade on a 2.5% grade. (Are there any changes to basic train physics between 09 and TANE? Or was it just my rather lousy driving of the helper?) And it seems shadows look pretty bad on steam engines at anything less than high, mostly due to "self shadowing" issues. (Can self shadowing be disabled?)
When it comes time to run Rollins Pass, which is rather scenery-sparse in the high Rockies (pass elevation 11,500 feet), I'm hoping for a 10 km draw distance, and high tree/scenery draw distance. I hope. (I'm tailoring graphics settings to the type of route.) We're good here!
One other thing - when making a freelanced route, separating towns by greater than the maximum draw distance is a pretty good idea, right?