Richard J. wrote:

No, I don't think it has. In my experience, people used "railway" or
"British Rail" or "main line" (even when talking about purely suburban
railways) or the name of the BR/NR operator. People certainly didn't talk
about "London overground" even if they might possibly have used the word
"overground".

Maybe, but if you spoke of the "overground" most peple would think you meant
the "railway". Whilst the term wasn't used that much it was clear what it
meant until now.