well you can only go 3 times each to the same rail, after that you need an extra rail by either CB or OB.
hopefully Bob Jewitt will chime in on this one !
-Steve
Under WPA rules:
Prior to the 1/1/2008 rules revision, it was almost the way you describe. Essentially (with some other details), the limit was twice for each player. The guy who started it had to get the extra rail on his third time or it was a foul.
But this was changed effective 1/1/2008. Now, there is no limit in the rules on how many times the OB can be sent to the same rail. However, a Stalemate rule was added to the 14.1 rules, referencing the General Rule on stalemates:
4.12 Stalemate
If a stalemate occurs (see 1.12 Stalemate), the players will lag again to determine who will shoot an opening break.
1.12 Stalemate
If the referee observes that no progress is being made towards a conclusion, he will announce his decision, and each player will have three more turns at the table. Then, if the referee determines that there is still no progress, he will declare a stalemate. If both players agree, they may accept the stalemate without taking their three additional turns. The procedure for a stalemate is specified under the rules for each game.
Edit: Sorry, Bob. I was composing my response while you posted yours.