Potting balls frozen to the rail can present a challenge and drills get boring real fast. I made up a game to practice rail shots, its called rail ball.
There are no BCA rules for it so if you have a what if question, make it up, its your game, your rules. Basically place all the balls as shown and start shooting. Position is fun on this game and almost natural, at least from the first shot, then its up to you. Its also fun to look at peoples expression at the pool hall when you set up the table since it looks like nothing they have ever seen before.
I had not played it for a while and just did it recently due to a degradation in my rail shots and when I was done with a couple of games I was close to running the rack, rack of rail ball.