There are other issues which might affect the speed, too, though since you don't specify a route, a locomotive consist, or how much of whatever products you are carrying, I can't tell whether they would affect your situation, or not.
In Trainz, as in the real world, the amount or power per ton of train affects the speed. If the train weighs a certain amount, then it will require a specific amount of power to achieve a particular speed, and the higher the particular speed, the more power required per unit of weight of the train. So, while you may see a low powered switcher powering a long train in a yard, it is not going very fast. And if the train is going up a grade, it requires more power to achieve the speed than traveling on the level.
So, in the absence of more complete information, it cannot be ruled out that 40 mph is the maximum speed your train can go with the conditions on the route.
ns