Chris J Dixon wrote:

Until the advent of power electronics in rail traction, then control
was generally either by tapchanger, for ac supplies, or series
resistors for dc schemes. In both cases the steps (or notches as they
are known) are finite, not continuously variable, and there will
always be jerks.

On more recent trains, I seem to hear the electronics "whistling" a set
of steps as they depart, why not continuous?

Or are those just more noticeable and the latest ones /are/ continuous?