Which matters, because Doctor Yellow inspection trains can be put into the schedule with the regular trains. There's no need to shut down traffic while a slow inspection car chugs along.

BART recently got a full-speed inspection car.[1] They needed a specially built one because BART has a non-standard track gauge.

The Federal Railroad Administration has track inspection cars, but only six of them for the whole country. One was seen on CALTRAIN track.[2]

[1] https://www.youtube.com/watch?v=GHa1Si7CW8Q

[2] https://www.youtube.com/watch?v=RQcRHn9YiPg