The rate at which the decoder decreases from one speed step to the next in response to a new decrease speed command. Use CV04 to set up the deceleration value for each decoder. This feature is used to simulate train momentum so that, just like the prototype, your model won't stop immediately when the brakes are applied. Decoders, Mobile: the electronic device installed in each locomotive that receives the signal from the command station through the track, decodes it & tells the loco what to do. Decoders, Stationary (Accessory) : the electronic device for the switches or other accessories that receives the signal from the command station through the track, decodes it & tells the switch/accessory what to do.