BART Trains Testing at 100 mph+

The BART revenue fleet was designed for a top speed of 80 mph, but the cars have exceeded this speed during various points in history.
 
From time to time, the legacy fleet ran at 83-84 mph under automatic mode during revenue service, typically while going down the grades in the Transbay Tube with smaller wheels (closer to 28 inches as compared to the 30 inch new wheel diameter).
 
Even bolder records were under manual control with the governor jumpered allowing the car to reach whatever speed the motors would take it. These were often done with only a single car, and of course done outside of revenue service.

Early 1970s:
There are rumors of a BART train reaching speeds of up to 110 mph, or more, before the motors blew out, during the early 1970s. Unfortunately, the details of this early 1970s test are likely lost to time, but the test, if it occurred, would have likely happened on the A line, around Hayward or Fremont.

Hayward Test Track
Additionally, old timers remarked that a car, ungoverned, once reached 120 mph on the Hayward Test Track. Another Train Operator notes his record on the test track as 93 mph with a single car.
 
With this in mind, the BART car was almost certainly the fastest urban rapid transit vehicle in the United States, beating the confirmed 87.75 mph record of the New York City Subway’s R-44 cars by a country mile.