Many thanks for a good trail. I only hike on it and probably won't bike it much unless I get a different bike or fork...
Just to add a note from 2011, this trail now has a 4-way intersection with a new (or maybe rebuilt from many many moons ago) trail that takes a level line across the south side of Mt. Watson. Hopefully this description makes sense.
So let's say you're on the Tahoe Rim Trail eastbound, like going clockwise around the lake. You ride over Painted Rock, then cross the Fiberboard Freeway and continue on singletrack a quarter mile, then you're on reclaimed logging road for a bit. You pull a hairpin right turn to avoid hitting the Fiberboard again and come out at a killer lake view where there are posts to prevent vehicle access further east. You continue on reclaimed logging road that is starting to narrow to quasi-singletrack over the years as trees and brush grow in and logs fall. Continue eastbound until you reach the intersection where the Rim Trail takes a fairly steep left up the hill to get on up and over the shoulder to Watson Lake. Instead you continue eastbound fairly level. This has always been the way to Elevator Shaft (and the trail mentioned by Silverback). Used to be your only options from there (although there was some flagging that I followed a couple of times continuing level and eastbound... but all I got was a lot of scratches).
So. What's new is that the level, eastbound trail has been fully constructed, singletrack, *not* reclaimed road, all the way across. There is some hike-a-bike where it gets rocky before and after a creek crossing. Not too long after the creek crossing is the intersection with Silverback or 3 Arrows Down. Anyway, we continue east and still very level (with hills, of course, but kind of keeping on an average altitude). The best part of the trail is the next mile where it's beautiful singletrack with just enough swoops and obstacles. Then you cross another small creek and come out at forest road 49 (i think?).
I could write a lot more encouraging words for all the loops you can make out of this, but maps will do a better job some day, just not today.
The main thing, though, is... where it used to be that if you continued on the Rim Trail from Painted Rock eastbound, you would be forced to choose between steep gnarly downhills to Burton Creek and TC High School and the X/C area, or going all the way up to Watson Lake. Either way you had to choose between too short and too long of a ride with not enough loop possibilities. Now there is a non-Rim-Trail alternative that lets you continue eastbound from the Watson area without having to hump that hill all the way up to Watson Lake... makes about four more really good loops possible.