Window glass.

But doesn't all auto glass have to meet strict government standards for safety????
Definitely, but I think part of that is the glass has to break easily without any glass shards in the event of an accident; in example, someone going through a windshield. It is also a part of integral strength of a vehicle. Don't quote me on any of that. Those are just my old memories that I think are correct. Lol. And locomotive glass is 9/16" thick thick, so that's a different animal. :)