Its not really a guide, but its how the cars are trailored.
Next time you watch a Floorless or Invert video, watch the front 2 cars VERY carefully. They will not bend into each other nearly as much as the cars behind. This does have some impact on smoothness, but that smoothness is defined by vibration. Im not 100% sure but I remember a debate and someone had the great point of what I just said plus pointing out that not trailoring the cars as such would produce a very unstable train or something like that.
that front car is also called a Zero-Car, since the first car is always the first row with seats.