This works on a custom scenario also. I have set one up on the track of Schnellfahrstrecke Köln because this one is quite long (~65km) and has some higher speed limits. Start station is Aachen and end station is Köln. Time is 3am. No other trains, no red signals from start to end.
After setting up the locomotive, just set full power (notch 8) and then save the game. Notch 8 will give you a speed of about ~125km/h which is fast enough for most of the track (except the beginning and and stations and some 100kmh sections). Once you are at the end of the map, you will run a red signal and the game ends. Then simply restart the saved game and you will enter the map already driving.