In the mid 1960's (before '65, I didn't have a DL yet) not far south of San Francisco on the SP, saw a train that really suprised me. It was a coal train headed south. Locomotives, every car and caboose were Rio Grande.
I tried to sell my two cents worth, but no one would give me a plug nickel for it.
I don't have a leg to stand on.
The OP did mention back in the 60's or 70's. During that time frame, D&RGW loco's rarely, if ever, wandered off-line. In my Rio Grande Diesels book by Joseph Strapac, he does have a few photo's of D&RGW loco's in California but IIRC, those were from the early 80's in the years just prior to the book being published.
As the 80's wore on, there were instances where SP loco's ran east into Denver and a few Rio Grande engines wandered west. After the D&RGW purchased the SP in 1988, then there was a lot more mixing across the new combined system.
Rio Grande. The Action Road - Focus 1977-1983
I am have not visited Tehachapi but, if you want to mix D&RGW and SP power on a model of the loop, I would opt for post Rio Grande takeover. This is not to say Rio Grande NEVER appeared on the loop but, to work in GP-30s, 35s, 40s, 40-2s and 60s, as well as SD40-T-2s, 45s and 50s, post takeover is it.
Due to Rio Grande's locomotives being in generally better shape than SP's, they began showing up anywhere SP went. Prior to the takeover, it is possible D&RGW power did traverse the loop on run-throughs such as coal trains, or working off horsepower hours owed SP, for whatever reason. This is doubtful because the San Antonio Division, Employe's Timetable No.7, effective Oct. 30, 1977, lists maximum speed and lengths by equivalent SP locomotive, for BN, B&O/C&O, Conrail, Colorado & Southern, L&N, N&W, Rock Island, Seaboard Coast Line and, Southern-but no Rio Grande. Maybe someone from CA could fill in the blanks as to why no D&RGW. It is also odd that western competitors Santa Fe, WP, and UP are also among the missing although, in the 1980s, all were regular visitors to SP rails east of El Paso.
Appearntly the Rio Grande purchased Southern Pacific. Southern Pacific was the owner of the Tehachapi loop in the 60s. It wouldn't be too surprising if a Rio Grande locomotive togo through Tehachapi.
There are still some odd locomotives that go through the Tehachapi loop. Occasionally a Norfolk Southern or a Candian Pacific locomotive will be seen.
I thought about doing the Tehachapi Loop. Because when I was a kid we were going back to Oklahoma to visit. We were going over the Tehachapi pass to get to the old Route 66. This was back in the late 60s or it could have been 1970. Anyways I remember seeing all these long lines of trains on the Tehachapi Loop. One thing I could have sworn that I saw was the Rio Grande locomotives using the loop. I could be wrong and seen the Rio Grande locos at some other point but I know that is when I fell in love with those trains. Does anyone one know if the Rio Grande RR ever used the Tehachapi Loop? If they did I want to do the loop using the Rio Grande trains. Thanks for any help anyone can provide.