Keith Dysart wrote:
Can photons explain the state of a transmission
line driven with a step function after the line
has settled to a constant voltage?

Of course, photons can be used to explain all
EM wave action. A step function accelerates
electrons which then emit photons as EM waves.
Hint: electrons cannot move at the speed of
light. EM waves move at the speed of light.
--
73, Cecil http://www.w5dxp.com