Seriously, how do folks discover things like this? It seems as though they must spend much more time seeking errors than operating trains, but why do so? I'm just curious.
You've never looked at the underside of the train in TSW before? Have you never watched the way the bogies move or the way the train rocks around on its suspension?
If you want to see how train rocks on its suspension, you should play DRS. In TSW all you can find under cars and locos is some fakeish bogie animation that doesn't react to any track feature except curvature, shock absorbers moving through their mounting points, non-functional axle suspension and wheels clipping through rails. Cooling and sanding hoses are bending nicely though.
I noticed the bogie issue on a 166 purely by coincidence. I was driving it, looked at it from the outside, and thought that something looked off. This made me curious, so I looked more closely at it. When I'm playing TSW I'm not looking for bugs, I simply notice things that aren't feeling quite right. Sometimes they are are genuine bugs, other findings may be like that by design.
I just waited in the station and had a look around, then i noticed the shadow coming out from underneath... Nothing special.. But since i program myself i always like to checkout how others do stuff.