Photos of the CONX tank cars in C&S NG trains are pretty rare. Besides the one linked above, I know only of these:
Near Grant, dated 1929. Otto Perry photo. Black tank car in the early lettering scheme.
Grant again, also 1929, Denver Water Board Special photo.
Waterton, 11-1935, Otto Perry. In the silver tank car era.
Baileys, clean-up train, April, 1937, Otto Perry. Final black CONX scheme.
FWIW, the sparse photo data might suggest:
1. The South Park crews liked to bury the tank cars in the consist of the train, keeping flammable liquids away from both locomotive and caboose, to better protect the crew.
2. The position of the tank cars in the train is all coincidental, from insufficient sample size of photos.
3. If the C&S NG had contracted with CONOCO to run empty tank cars all over the South Park, for advertising purposes, perhaps the contract included "framing" the tank cars between red freight cars for a better optical effect.
On the isolated Leadville/Climax segment, after 1937, there is this photo of tank cars directly behind the locomotives:
As the train is arriving in Leadville, the tank cars are likely empty, their volatile contents having been unloaded at the Climax mill.
Other photos show that CONX tank cars delivered petroleum products to various branch locations, like Alma, Black Hawk and Morrison. Unless they were set out empty, as static "bill boards".
Seems to me that I've seen some Clear Creek photos with a CONX tank car, usually number 8, both in the middle of trains and directly behind the locomotive. But Clear Creek is not my forte . . .
Jim Courtney
Poulsbo, WA