As several people have said, American RRs in general didn't buy Garretts largely as a matter of Not Invented Here.
As to the South Park, C&S never bought a new 3' ga engine. AFAIK, they never seriously considered it.[1] And they got rid of the last Mason on the line (C&S 1, nee DL&G/DSP&P57, nee DSP&P 24 Buena Vista) as quick as they could.
hank
[1] Yeah, I know about the studies done re the Uintah engines & other types in the mid-30's but those weren't serious, they were just a CYA operation because people had pointed out the lack of newer power at various abandonment hearings ca 1930 and made them look bad.