A tunnel through the San Gabriels, like BOB2 has advocated for in the past, would effectively be a base tunnel even though the altitudes at each end would be significantly different. That's because the land at each end is at the base of the mountains, so it's impossible to go deeper. Palmdale is at about 2500' elevation; Sylmar and other spots along the western base of the mountains in the San Fernando Valley area at 1000-1500' elevation.

A Tejon tunnel would not be a base tunnel, because the line would surface at the top of the mountains in order to cross the San Andreas Fault. It would be, at minimum, two tunnels: one from the San Joaquin Valley floor to near Gorman, and another from the south side of the fault down to Santa Clarita.

The last study I saw for HSR between Palmdale and the San Fernando Valley still had several alternatives, including one with multiple tunnels and lots of earthwork that more or less followed the current Metrolink line. IIRC, at least one alternative involved a long tunnel through the San Gabriels.