At SFGAm, and I assume other Six Flags parks...
At closing, they will close the line. The ride will continue to operate as normal until the line reaches the point where there are only enough people in the cue to fill one train. At that point, you lose your ability to wait for the seat of your choice, and they will ask you to choose an empty line so that the are not running the train totally empty for hours while everyone waits for the first and last seat. When it is getting close to this point, they will figure out how many trains they can fill and announce that there will only be 2 more trains, or 3 more, etc...
I must be lucky, because last time I was there I rode the last train of V2 in the front, and the time before that, I rode the back seat of raging Bull on the last train. These are the only 2 times I've been at the park during closing this year. :)
I have to add, that riding the front seat of V2 is a must ! There was some debate over which is better - Raging Bull or V2. After riding the front seat on V2, I can see why some people think it's better than bull. Wow it is amazing !
(Let's please not start that debate again - they are both wonderful rides).