They ran rod engines on the lower half, El Prado to Auberry, and geared engines from Auberry to Big Creek. Geared locomotives changed from Shays to Climaxes.
I've driven the upper half from Big Creek (Cascada) to Auberry. It makes you wonder how the RR got around the curves.
"The Railroad That Lighted Southern California" is the book to get for more information.
Map of the upper half:
Bruce