Classical-mechanical models without observable trajectories and the   Dirac electron
  We construct a non-Grassmann spinning-particle model which, by analogy with quantum mechanics, does not admit the notion of a trajectory within the position space. The pseudo-classical character of the model allows us to avoid the inconsistencies arising in the quantum-mechanical interpretation of a one-particle sector of the Dirac equation. 
A. A. Deriglazov
hep-th math-ph math.MP quant-ph