This is actually a fun one to point to. Replacing the Eastern span of the Bay Bridge happened recently, and has been one of the most expensive Californian infrastructure projects to date. It started with a price tag of $250 mil but it ended up costing $6.5 bil. In the meantime, BART has performed modifications on their tracks and has begun switching to new rolling stock, but it's been a lot cheaper than replacing the Eastern span of the Bay Bridge.