So, I was undertaking one of the journey related scenarios and decided that the coach lights needed to be on. you can also see that the light doesn't just spill through the roof, it also leaks out through the chassis Now, if I recall correctly from my model railway... DTG need to put some silver foil into the roof (and also the floor) to stop this error...
Happens with the german Dostos as well. Certainly not very beautiful to see the light going through the roof!