Wow! I haven't seen this yet and I'm really surprised to see this realistic feature! I wonder why DTG hasn't told us about this incredible feature and waited until we discover it ourselves. Anyway: Congratulations, DTG, really good idea!
Well, Matt is a lead developer on TSW so in a roundabout way they have told us about it. :P However, I suspect it hasn't been mentioned much because it may not yet work with rain and snow. Actually, that would be an interesting test; take a Class 66 or a CSX loco and connect a heavy consist during heavy rain and see what happens.
Wheelslip is impossible unless you do crazy stuff like that. The adhesion to the rails is basically 100% everywhere all the time no matter what. In real life, even in good conditions, adhesion is nowhere near 100%. In rain and ice, adhesion is like nothing. But that's real life, not TSW.
