It was noted elsewhere that getting the Class 166 up to 90mph is a struggle at best. I noticed tonight during journey #4, Redding to Paddington, that feathering between power 6 and 7 seems to help a little. I'm submitting that part of the issue is due to wheel slip under icy winter conditions. Too bad Euro engines seem to lack wheel-slip indicators as on some American power units. Try feathering that throttle, fellow engineers, I'd like to compare notes.
I think the 166 is hard-limited to 90mph. Even downhill with a following wind and the throttle wide open it won't exceed this figure (unless driven by AI, which of course doesn't really drive the train, it just moves the object around the map at an arbitrary speed). If you're having trouble even getting to 90, then yes, lack of traction could be the issue.
I've never had a problem with Class 166. To me, it's one of the most easily operated trains at TSW. I have long since completed GWR, everything, so I don't know how it is now but I'll try, at least a few more times in all weather conditions.