California high speed rail has been a mess. Which is likely in part because politics dictating routes has raised the costs and timelines substantially. On a smaller scale SF was building a new subway in 2013 that had been on the drawing board for years. I remember thinking maybe I'd ride it to work one day. Opened in 2023 or 2024, after I had moved offices twice and then went to work from home. It's not a terrible line but because it had to go to the center of our Chinatown instead of 2 blocks over, it took quite a bit longer and became the deepest subway line in the city. Several other bits of stupidity too in that project but a big piece of the delayed timeline was that tunneling in SF is hard.

Plenty of other transit projects exist that have made real differences. Personally I'm a fan of the simple improvements: revised bus routes with dedicated bus lanes and improved stop & shelters, added bike lanes, etc. those sorts of projects are relatively cheap investments and while no single one is a silver bullet they add up. On a bigger scale - Caltrain's electrification was a big win. Both kinds of projects are easier than building whole new tracks or digging new tunnels. Extend and improve the existing systems. Most cities have something to start from.