  The generalized Dirac equation of the second order, describing particles with
spin 1/2 and two mass states, is analyzed. The projection operators extracting
states with definite energy and spin projections are obtained. The first order
generalized Dirac equation in the 20-dimensional matrix form and the
relativistically invariant bilinear form are derived. We obtain the canonical
energy-momentum tensor and density of the electromagnetic current expressed
through the 20-component wave function. Minimal and non-minimal electromagnetic
interactions of fermions are considered, and the quantum-mechanical Hamiltonian
is found. It is shown that there are only causal propagations of waves in the
approach considered.
