I just came across this in the rules looking for something else this week. If it leaves the rail, hits another ball, and then returns to the same rail, it's a good hit. If it didn't hit another ball, its a foul.
I think you were looking at the BCA PL rules. There is no dead rail under the WPA rules.
WPA
2.7 DRIVEN TO A RAIL
A ball is said to be driven to a rail if it is not touching that rail and then touches that rail. A ball touching a rail at the start of a shot (said to be “frozen” to the rail) is not considered driven to that rail unless it leaves the rail and returns. A ball that is pocketed or driven off the table is also considered to have been driven to a rail. A ball is assumed not to be frozen to any rail unless it is declared frozen by the referee, the shooter, or the opponent. See also Regulation 26, Calling Frozen Balls.
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So under WPA rules all it has to do is leave the rail and come back.
Under the BCAPL rules, it has to leave the rail, hit a different rail or leave the rail, hit a different OB (not the CB) and return to the rail.