Window glass.

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. :)
The way I see it, with all due respect, the thickness of the plastic is a big factor on how well the glass holds up during impact? The thinner the plastic the easier the glass gets destroyed? I may be wrong???