The California High-Speed Rail project is plagued by inflation due to being a multi-decade project, land acquisition issues related to eminent domain, abuse of CEQA, obstruction by railroads against sharing or even crossing their right of way, utility relocation delays, and project's early reliance on consultants instead of growing their own in-house expertise from the beginning.
Not much can be done about inflation except completing the project more quickly. Land acquisition and environmental clearances are mostly done. I expect more friction from Union Pacific for the segment from Gilroy to San Jose, and utility relocation delays also continue to be a problem. The project has grown some in-house expertise, so future extensions should be cheaper.
So you see, Republicans still have a lot of power to obstruct the project and drive the cost up, and then they blame the Democrats for it.