Responsive day/night coaches
From TrainzOnline
Most of our passenger coaches support responsive day and night light setup which means that the light is turned off on daytime and you don't have emissive in the model. That creates a more realistic appearance. Once its getting dark the light will be enabled. This setup allows you more realistic session creation as you are able to place unit passenger coaches in yards or you can change the behavior yourself.
Disable Light
To disable the light just open the properties tab and select NAME OPTION HERE. The light will be now off by default.
Toggle in-game
To enable or disable it in the session you need to download the driver command Interior Lights Command <kuid:447264:1043> and apply it as you wish.