The entities take the light value from the "ground", which is the worldspawn (level's static geometry). Stuff like trains, doors, buttons (func_#### and similar brush models) aren't considered "ground" because they aren't worldspawn.

This video should help:

https://www.youtube.com/watch?v=u8-mw79NGDU 

Is it possible to cahnge the value of gravity during the level? e.i. I'd like to have low gravity in one room.

I also forgot, how to put dead bodies on the map. I remember killing monsters somehow, but can't quite remember the optimal way. 

hypnotic and some other mods have triggers that set gravity on the player so you can place those triggers at all the doorways where gravity needs to change. (should be pairs of triggers so that it gets reset if you go back out.) Entities that spawn inside the room need their starting gravity set explicitly using “gravity” field on each entity. 

I wanted to slow down the fall of the player. My initial thought was to make an invisible moving platform, but it worked awful and broke my level. Then I tried trigger_push (pointed down), and it did the trick beautifully.