Hi Tim,
The turntable was moved to the location shown on the map that you posted after the fire that burned down the Stone Engine House across from the Telegraph Office and Boarding House.
As far as the other grade goes that you pointed out...
Although this is just conjecture on my part, I think that it was a combination of the following:
1. This track was built far earlier than the replacement turntable. When I checked some of the early photos of the west Portal area, there was a photo published in (I believe) the Mineral Belt V II of a Mason Bogie and car descending from West Portal toward the Engine House. If you examine the photo, you will see this track in the background.
2. I think that this grade was built from the waste generated by digging the West End of the Tunnel.
3. The track, although most maps and records from that era are lost to time, seems to be a "safety" or "runaway" track meant to slow down or intentionally wreck an out of control train descending from the Tunnel to the station. There are examples of these on the Colorado Midland and I see the remnants of one each time that I pass Borie, Wyoming, which is at the east base of Sherman Hill. Old Rails used to refer to these as "Go to Hell" tracks, because if you needed them, you and your train would...
As I said, this is just conjecture on my part, but based on my experiences as a railroader. I know that the railroads didn't build something just because they thought that "It would be neat".
Rick