If you intend to switch between cabs or locos on the fly without speed interruption, the regular Cab06 is the one you want.
The P version is a potentiometer; it has stop and start points like an old DC power pack. Smooth in between a definitive start and stop.
The standard digital encoder version has no start or stop point. The knob spins continuously, with small microindents.
A potentiometer or "P" type cab is an ABSOLUTE speed control. Its position indicator provides absolute feedback on speed and position relative to the stop and start points. It is the same type of control as a volume control one would find on an electric guitar or an older stereo system. So if you set the knob halfway between stop and full, you will have half speed.
For running a single train, the P throttle is easy to understand and use, since it works like a DC throttle for speed control. Do not use this type if you want to change trains without stopping the current train you're on.
If you are running train A at half speed and then select train B, which is stopped, train B will suddenly take off and accelerate to half speed as quickly as the decoder allows. Likewise, if you stopped a train you have selected, set the throttle down, and unplug it, but accidentally knock the throttle knob to a non-stop position while it was unplugged, then plugging in the throttle will result in the train suddenly moving at the speed set by the knob without you doing a thing. One of the mysteries people encounter when they turn on the layout after it was shut down, and are surprised to see a train suddenly moving when they turn the power back on, can be traced directly to a "p" throttle knob not in the stop position.
Bottom line: controlling two moving trains at the same time becomes very difficult with a "p" throttle, and you must be careful with the throttle, or you will falsely think you have a runaway train.
The regular encoder cab is a RELATIVE speed control. There is NO position indicator or feedback on the speed position, since there are no stop or start points. The train's speed is tracked only by the command station. Turning the knob up or down increases or decreases the current speed, respectively. So if a given train speed is currently zero (Stopped), then turning up the knob will increase the speed above zero. Keep turning the knob and more increase in speed.
Likewise, if a given train speed is NOT zero, turning the knob up or down simply increases or decreases the current speed, respectively. Keep turning the knob up and more increase in speed. If you choose to turn down the knob, the train's speed will decrease.
The advantage of the normal encoder cab becomes very apparent when you switch trains.
Example: If you are running train A at half speed and then select train B, which is stopped, train B remains stopped while train A continues running at half speed. You can flip back and forth between two or more trains of different speeds and never have to worry about sudden speed changes. If you stopped a train you have selected, set the throttle down and unplug it, but accidentally knock the throttle knob while it was unplugged, then plugging in the throttle will NOT result in the train suddenly moving.
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