So I have a radio scanner and sometimes I park my car outside of train yards and listen to the chatter. From all I've heard I think it's best to phrase it as "XXX Tower, track XXX is ready for a lineup". That's if the little short leaving signal is red. Once it goes to yellow then you say "XXX Tower, permission to proceed on track XXX". I practice this all the time in the simulator I built in my basement and I've never had an accident, simulated or otherwise.