Why is proving $C^{\infty}$ regularity of sub Riemannian geodesics so hard? In Montgomery's A Tour of Subriemannian Geometries, Their Geodesics and Applications, problem 10.1 in Chapter 10 asks "Is every minimizing geodesic smooth ?". 
Can someone explain what are the major difficulties faced in proving the same ? Suggestions for suitable references are also welcome. Thanks.
 A: According to the MathSciNet, the first general contribution to the solution of the problem is given in the following paper:
E. Hakavuori, E. Le Donne,
Non-minimality of corners in subriemannian geometry.
Invent. Math. 206 (2016), no. 3, 693–704. 
(MathSciNet review).
The paper mentioned by Richard Montgomery is:
E. Le Donne, G. P. Leonardi, R. Monti, D. Vittone,
Extremal curves in nilpotent Lie groups.
Geom. Funct. Anal. 23 (2013), no. 4, 1371–1401. 
(MathSciNet review).
A: The reason the problem is hard is that we do not have  a good handle on what abnormal (=singular) geodesics can look like.  See the chapter of my book that describes  abnormal geodesics.  Progress is being made in the Carnot case, but it is slow.  
I would look at a fairly  recent article 
(EXTREMAL CURVES IN NILPOTENT LIE GROUPS
ENRICO LE DONNE, GIAN PAOLO LEONARDI, ROBERTO MONTI, AND DAVIDE VITTONE; I think in GAFA)  on the Carnot case for state-of-the art stuff, and the first paper by myself and that of
Liu-Sussmann just to get a feel of abnormal geodesics. 
It is POSSIBLE the problem can be solved without understanding abnormals - by some direct analysis.  No one has done anything like this: the abnormals always seem to be there, hiding.
