The Rail position can be very important. 95% of punters don't check it, 95% of punters don't adjust for it, 95% lose.

Looking at the rail position can give you an advantage over punters who don't take the time to check it out.

But more to your question ... It depends on the track and where the Rail is out from. If its just out in the last 200m then probably not, but it will restrict running room for the horses trying to find a way through.

The rail position obviosuly affects horses behind the first 4-5 horses more then it affects those that lead. So the impact may be negligble depending on the running position of your horse.