It’s an indicator light for waste heating system.

This photo was taken in a train with a vacuum toilet (https://en.wikipedia.org/w/index.php?title=Toilet&diffonly=true#Vacuum_toilet). These toilets need less water for flushing, but since the waste is stored in a low pressure environment you really don’t want it to freeze and break anything. That why it’s heated.

Also, the proper conjungation in this case is “fekaliów”, not “fekalii”.