What happens when I select a loco that is currently running at a different speed than the throttle is set to?
example... running loco 'a' at half speed then switch to loco 'b' does loco 'b' suddenly change speed to half throttle?
(which is the last position the knob was indicating for loco 'a' this would make for a nice crash if I planned on backing loco 'b' out of a siding, and it goes forward instead at 50% right into a bumper)
You're right, when switching the loco 'b', the newly selected locomotive would be commanded to operate at half speed.
In practice, operators have developed a series of techniques to deal with this.
The easiest seems to be punching in an address which is not being used, turning the throttle down to zero and then calling up the desired locomotive.
Another approach involves quickly punching the DT400 throttle knob to reverse the locomotive and then quickly turning the speed knob down.
With the Zephyr, you can put the reverse handle into "Brake".
If worse comes to worst, there is an emergency stop function available with all throttles.
Typically, there is a small amount of lag time between when a locomotive is selected in this manner and things actually begin to happen.
The amount of lag time is related to how much activity is present on the system.
If there are a lot of trains in operation, the system's polling time between locomotives may be longer than if there are only a few locomotives in operation.