  A number of recent Molecular Dynamics (MD) simulations have demonstrated that
screw dislocations in face centered cubic (fcc) metals can achieve stable
steady state motion above the lowest shear wave speed ($v_\text{shear}$) which
is parallel to their direction of motion (often referred to as transonic
motion). This is in direct contrast to classical continuum analyses which
predict a divergence in the elastic energy of the host material at a crystal
geometry dependent `critical' velocity $v_\text{crit}$. Within this work, we
first demonstrate through analytic analyses that the elastic energy of the host
material diverges at a dislocation velocity ($v_\text{crit}$) which is greater
than $v_\text{shear}$, i.e. $v_\text{crit} > v_\text{shear}$. We argue that it
is this latter derived velocity ($v_\text{crit}$) which separates `subsonic'
and `supersonic' regimes of dislocation motion in the analytic solution.
  In addition to our analyses, we also present a comprehensive suite of MD
simulation results of steady state screw dislocation motion for a range of
stresses and several cubic metals at both cryogenic and room temperatures. At
room temperature, both our independent MD simulations and the earlier works
find stable screw dislocation motion only below our derived $v_\text{crit}$.
Nonetheless, in real-world polycrystalline materials $v_\text{crit}$ cannot be
interpreted as a hard limit for subsonic dislocation motion. In fact, at very
low temperatures our MD simulations of Cu at 10 Kelvin confirm a recent claim
in the literature that true `supersonic' screw dislocations with dislocation
velocities $v>v_\text{crit}$ are possible at very low temperatures.
