Warning system works is such a way that your warning level is decreased each day(excluding days when your warning level rised) by 5%. You get 20% of warning when you derail due to over speed and only then. I suggest you to enable speed hits witch will show you that you have to brake in advance.
Driving on PL1 is blocked when you exceed 40% warning level, so you should be able to play next day when it drops to 35%. But if you then derail you will have to wait a little bit longer.