Are you a 2-Rail modeler or a 3-Rail modeler? It makes a difference whether you use a plastic gear box if you are a 2-Rail modeler, or a metal gear box for 3-Rail modelers.
Huh? What does the number of rails have to do with this? I use them - 804 or 8-05 - interchangeably w/o any concerns or worries regardless of what the car is made of or number of rails for that matter. The only time I might care is to add a bit more weight to a wood car and then grab a pack of 805's.
Because metal boxes with metal frames on 2 rail track equals a dead short...hence the purpose of the plastic boxes.
Only if you let it happen, 
Not a problem I've ever run into with metal cars.
some wheel sets prevent this from happening. Maybe U have those? Most NWSL wheel sets that I have, are only insulated on one side. So if the car happens to be put on the track a certain way and hooked up to the next that's on in reverse,...... snap! I've gotten shocked playing the switching game by hand!
Now, all my equipment gets plastic boxes! Don't want to loose any electronics over stupid stuff like this.
