Howdy -
My favorite ride in the area is Sprague River Road from Chiloquin to Beatty - it's a great paved county road with very little traffic and small farms and ranches characteristic of the high desert. Hwy 140 from Beatty to Lakeview is a nice high desert route with light to moderate traffic.
If you are getting off the train at Kalmath Falls you can go any direction from there. Consider doing a loop that includes Crater Lake and points east. Don't know how long you have - but Medford is awfully hot in late July. Are you riding on mountain bikes? Or can you do some dirt? The road on the east side of the Warner Mountains in NE Calif is super - you could head down to Cedarville and then across the mountains back to Alturas. Hwy 139 back towards Klamath is rather dull - it goes through lava flats - fairly level - with thick forest. The back road thru Lava Beds NM and Tule Lake NWR is nice.
The mountains are great in late summer but you may have forest fire problems given the drought that southern Oregon is facing. Hwy 140 is steep and quite busy, Hwy 66 is better, Hwy 62 is a nice run along the Upper Rogue until you get close to Medford when the traffic really picks up. Hwy 138 is really sweet - you can stop for a swim at Diamond Lake where the top two inches of water are warm by early August.
Oregon DOT doesn't have the bike map on line - also they aren't printing as many given the budget crisis.
Here's a website with traffic volumes - next best thing - big file, though.
http://www.oregon.gov/ODOT/TD/TDATA/...flowst2003.pdf
Best - J