Caught a steam train that operates on the mainline in NSW, Australia last year.

https://www.picnictrain.com.au/

We had to stop & wait at various points on our return day trip for other passenger services to pass us. Had to wait for some freight services to clear a block at a few places too.

There was an auxiliary diesel unit not actively used apart from for power AFAIK (and additional emergency mobility should things go kaboom/excess wheel slip).

Maybe that has some the extra safety systems on it though.