Problem: The speed of the engine or trains changes when I turn the headlight on or off.
Answer: You command station is set to 14 Speeds step mode but you decoder is set to 28 Speed Step mode. Two solutions:
a) Go to CV29 in the decoder and change the 14/28 bit for 14 speeds step mode.
b) Set the throttle to work in 28 Speed Step mode.
If you wish to learn more about the background why this happens, go here: Mixing Up Speed Step Modes