This is the real reason legacy car makers don't want fully autonomous vehicles.
Tldr: It should be a wash.
The systems have suffered many failures and crashes over the years, due mostly to poor maintenance and gross mismanagement.
Now the agency is under pressure from government regulators to retire old cars because they are not crash worthy. I find it alarming that we would talk about rail cars being crash worthy. Why not improve the system so cars don't crash? Nonetheless, new cars are much stronger and more crash worthy.
Since there are many more possible points of failure with autonomous cars (bad roads, non-autonomous cars being out there too, objects in the road, bad programming like that involved in the Tesla crash, etc) autonomous cars are going to have to be at least as crash worthy as Metro cars for the foreseeable future.
Cars that do 400 miles a day will reach 200'000 miles within ~1.3 years. Usually most cars are fairly low value by then. They would reach 400 miles by being on a highway for ~6.5 hours/day, which is fairly reasonable to assume.