Mrfss, Here is kind of what I found. First, there were no through trains from SA to Denver. However, it may still be that you did not change cars--maybe you were on a troop movement and they moved your cars for you from one train to another, a through car arrangement for the military.
That could have been either a whole train or just one or two cars.
Anyway, here are some possibilties: Missouri Pacific SA to StLouis, change to MoPAc StL to Denver.
MKT("Katy") SA to St. Louis; MoPac StL to Denver.
Katy, SA to Kansas City, change from KC to Denver by Union Pacific.
Texas Zephyr went from Denver but to Dallas Forth Worth and Houston, not SA, though maybe you caught it some other city.
As to MoPac(and T&P, Texas and Pacific) going from SA to Forth Worth, it didn't really. It served each city, true, but did not seem to go between the two that I could find.