That 318-340 tender situation is an interesting one. The tender carried by the 318 was the original tender from the Florence and Cripple Creek and is the one that had the 'backwards' Rio Grande's. It has trucks that are very different from the 'standard' C-class tender trucks of the Rio Grande.
Because of a long stretch between water tanks on the Ouray Branch in later years (IIRC one of the tanks went out of service or burned), the little tender of the 318 was not of sufficient capacity for the branch. So they swapped the larger, approximately 3000 gallon tender on to whichever engine was assigned to the branch. In those final years that tender was assigned to the 340. It had previously been on the 345, being in use with this engine when it was on the C&S. It too had unique trucks, with visible coil springs. This, along with the missing beading on the raised portion of the bunker made it very distictive. I'm pretty sure it was given to the 340 because of her assignment as back up engine for the 318 on the Ouray Branch in the late '40s.
What is interesting is that the 340 ended up with the 318's little tender and 318 ended up with the big one, since she was the last engine in use on the branch. Up until the 1990's you could still make out the "318" under the paint on the rear of 340's tender at Knott's Berry Farm. The big tender is, of course, temporarily in use behind 346 at the Colorado Railroad museum.