  Aim of this paper is to prove the second order differentiation formula for
$H^{2,2}$ functions along geodesics in $RCD^*(K,N)$ spaces with $N < \infty$.
This formula is new even in the context of Alexandrov spaces, where second
order differentiation is typically related to semiconvexity.
  We establish this result by showing that $W_2$-geodesics can be approximated
up to second order, in a sense which we shall make precise, by entropic
interpolation. In turn this is achieved by proving new, even in the smooth
setting, estimates concerning entropic interpolations which we believe are
interesting on their own. In particular we obtain:
  - equiboundedness of the densities along the entropic interpolations,
  - local equi-Lipschitz continuity of the Schr\"odinger potentials,
  - a uniform weighted $L^2$ control of the Hessian of such potentials.
  Finally, the techniques adopted in this paper can be used to show that in the
$RCD$ setting the viscous solution of the Hamilton-Jacobi equation can be
obtained via a vanishing viscosity method, in accordance with the smooth case.
  With respect to a previous version, where the space was assumed to be
compact, in this paper the second order differentiation formula is proved in
full generality.
