London and Budapest had subways before Boston did. So did some other cities depending on which list you look at.

So what made Boston’s later entry the first “practical” one?

[Edit] Or do you mean self-driving subways? Does Boston have one already? A quick Googling suggests the opposite:

https://whdh.com/news/mbta-officials-considering-self-drivin...

Sure, but are they self driving?

A number of european capitals seem to have managed to do driverless high capacity underground trains. Here in the UK, we've got a number of automated trains but for union reasons they still have drivers in the cab who press go at each station.

In the US, it looks like Detroit has a self driving line, and there are a bunch of airport shuttles. Presumably you are hitting the same union issues as us?