When track power is on, the LED is on, and when track power is off, the LED will be off.

When the LED is on, with no analog loco being operated, the LED should be orange - some call it yellow. If running an analog loco at full throttle forward, the LED will be either green or red. When running it backward, it will be whichever color it isn't when running forward. The reason it can't be specified which color it will be when running forward and backward is because the polarity on the track dictates the direction of the loco in relation to the layout, not in relation to the loco. When the polarity is one way, the loco will go in that direction regardless of which direction the loco is facing. For example, if the polarity is such that the loco will go West, it will go West regardless of whether the loco is facing East or West. So, if the two screw terminals are connected to the track power so that the LED will be green when the polarity is set for west, it will be red when the polarity is reversed (to reverse the direction of the analog loco).

The LED does not switch instantly from orange to green or red. When the analog loco's speed is set for zero ("00"), the LED will be orange. As it is given throttle, the LED will gradually change from orange to red (or green) until it is at full throttle where the LED will be solid red (or green). Also, since this is a bi-color LED, with the color dictated by the polarity given to it, the color is only accurate when looking directly into it. If looking at it at an angle, it could look a little red or green when it's actually full orange.

This kit is comprised of a standard two-wire bi-color LED, appropriate resistor, and piece of shrink tube.