Despite the growing prevalence of driverless vehicles such as robotaxis across the United States, virtually every mass transit network still relies on a human driver or operator onboard.

Now, two major American cities are seeking to alter that pattern.

Officials in Washington, D.C., recently advanced a proposal that could allow computers to fully operate one of its subways, while Los Angeles is working to construct a completely automated line.

Honolulu already runs one.

The D.C. and Los Angeles initiatives, which will take years to implement and still confront financial and technical hurdles, arrive during an intense ongoing debate regarding the safety risks and benefits of automated rail and whether heavy initial upfront expenses justify long-term labor savings.

Supporters describe the effort as long overdue, pointing out that American municipalities lag far behind international systems that run unattended subways, metros, or buses, including several cities in Europe and Asia, alongside major networks…