My rule for counting the number of rails was simple: every time the cue ball touched a rail, even if it barely reached the cushion, it was a hit rail. I’m sure there were some instances where the player intended to play a 1-rail route and ended up going 2-rails, and vice versa. I’ve made the assumption that these close calls have canceled each other out due to the size of the study.

The pros averaged 6.01 position plays per game. The results for the number or rails hit is as follows:
 

Average number of rails per position play: 1.10

Position plays in which the cue ball does not contact a rail make up nearly a quarter (23.8%) of the pros position routes. The reason for this surprisingly high number is that they like to minimize the cue ball movement by taking advantage of stop shots and short-range position whenever a layout permits. It also shows that they excel at leaving shallow angles which permit them to take advantage of opportunities to play short range no-rail shape.

The cue ball must travel more in Nine Ball than any other cue ball. Still, the pros like to minimize its traveling distance as much as possible.  1-rail position, which makes up nearly half (48.5%) of their position plays, enables them to exert maximum control of the cue ball. 

2-rail position is favored by many players including Buddy Hall, who perhaps uses it best. 2-rail routes are used on 22% of all position plays. 3-rail and 4-rail routes, which are mostly extensions of 2-rail routes, make up 5.1% and .7% of the pros routes respectively. This translates into one 3 or 4-rail route in every 18 position plays, or about one every three games.

The results clearly show that mastery of 1-rail position play is the ticket to improving your game as quickly as possible. In the diagram below, 

Pro Versus Amateur Position
The example below shows how amateurs complicate matters by playing position for multi-rail routes when they are not the best choice. This mistake results from poor execution and/or poor planning. Pros, on the other hand, keep things as simple as possible. 

With the cue ball at Position A, the pro has an easy 1-rail route down Path A-1 to the 8-ball thanks to the modest 25-degree cut angle on the 6-ball. The amateur has made the mistake of leaving the cue ball at Position B, which is a 50-degree cut on the 6-ball. Now he must send the cue ball 2-rails across the table and back out down Path B-1to Position X.  The amateurs route will force him to risk scratching in the side pocket, In addition, the cue ball will be traveling across the position zone. The pro route sent the cue ball down the length of the zone, which makes it much easier to control the cut angle on the 8-ball.