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://en.wikipedia.org/wiki/Doctor_Yellow