Western Railroad Discussion > Tier 4 question

I apologize if this has been asked before, but today I got a close look at the side of BNSF Tier 4's and saw a very large sight glass on the conductor's side of the long hood just after the F of the swoosh logo.  The glass is about 2 feet long.  What is this for?  I assume it is not for urea levels since my understanding is that these do not use urea for removing pollutants. 

Thanks,

Paul
www.z-trains.com
Railfanjunction on Youtube.com

Coolant level for the engine.

Which is usually empty as GEs leak water all the time. The size of the sight glass is because GE has s screwy move all the water to the engine and then back to the tank thing going on. Getting the engine really hot helps in emission control.
Most times the engineer doesn't notice that his power is derating due to low water. G ood E nough.
One feather in the EMD cap. They hold water:)

And another observation they moved the rear numbers up to the radiator and use a different color to provide contrast.