TfL brought level 2 GoA to a few Tube lines. In the US newer subway systems are already at that level. DLR is GoA 3, and a few smaller lines in the US (e.g. Las Vegas Monorail, Miami's thing, pretty much all airport people movers) are Level 4.
Yes, newer ones don’t have the same problem, since they are not held back by the rules of a union. Sadly those are few and far between, and I haven’t heard of a new railroad line since the yet to be finished CHSR, though can’t blame that one on the unions.
BART and Muni are both union shops. I don't know if SMART is but it postdates the HSR proposals.
As someone who lived through Muni's transition to SelTrac (the system that underpins DLR and I think some of the Tube lines) I can say the union was not the problem. The technology was unreliable and eventually abandoned by Alcatel/Thales. GoA 4 stuff is only viable along small, low ridership stretches of track. If you knock the level of automation down a peg or two it's a lot easier to scale up to busier/larger systems.