The 166 in GWE can only get to 90 mph and no more, it just stops accelerating at 90, which is unrealistic for sure. No locomotive's rated speed is its actual maximum speed, especially in Britian where I've gathered that any train must have a cushion area of around 10% from its actual max speed to the rated max speed. If the TSW 166 were following those rules, it's rated speed would be 80 mph, not 90 mph. And in real life,a 166 should be able to reach around 100 mph. So for a bit of realism, and to make driving the 166 more interesting than "just floor it and let it be" the max speed it can reach should be increased.
I've seen an AI driver going flat out at 100mph when the speed limit was 125mph before now. I'll wait until after the update on 14 Dec before checking if it still happens.
