Details are here:
https://www.hsr.ca.gov/docs/programs/construction/HSR_13_06_B3_PtC_Sub1_CHSTP_Design_Criteria.pdf
But the short answer is:
Table 3-1:
Minimum Vertical Clearances
. . . . . . . . Item . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Minimum Vertical Clearance
Clearance from HST TOR to new structures . . . . . . . . . . . . . . . . . . . . 27’-0”
Clearance from HST TOR to existing structures (V > 125 mph) . . . . . . 27’-0”
Clearance from HST TOR to existing structures (V ≤ 125 mph) . . . . . . .24’-0”
HST tracks span over Road and/or Railroad (HST Overpass) . . . . . . . To be negotiated