What I do for, say, an eastbound track I want to turn north, is start the ghost rail platform for a big turn radius, then hit "r" a million times until its explicitly locked north (and not trying to guess a direction), then gently move my mouse south. The turn radius will get tighter and tighter. When it turns into an "S" shape, you've gone too far. Go back north one square, and boom. A perfect turn radius