If I’m not mistaken water jets is only used on some sections between 20 and 60 mph as below 20 mph it can damage the rails, but I’m not really sure about that information if someone can confirm.
The tutorial doesn't tell you to do that, unless it has changed significantly since I did it on release day. The above information is correct though, you must be moving at more than 20mph or you will carve a nice chunk out of the rails. You would also switch it off when passing over points and through stations.
Dad rail has done some videos on this as he drives them in real life. I think if I remember right you start them when you're moving although they have special metal plates to test them in the yard without damaging the rail. I think you drive max of 60mph but drivers irl can be told to slow down a little bit over certain sections if they're particularly bad. As I say Dadrails videos on the RHTT 66 are fantastic as he drives them in real life talks through real life procedures etc
