It's rather unintuitive as a new player to have the gradient display go from left to right, but the key to speed up is on the left (A) and the key to slow down is on the right (D). Maybe flip the display so the train is facing left so this is consistent? Because it's not at all intuitive to have to press the key on the left to go right. Hopefully the attached picture makes my confusion clear.
I can kinda see where you're going with this but the gradient display isn't anything more than it's name. It's not there to tell you which direction you're going, it's to show whether you're going up or down. I don't think people would use it or think it's for anything else. I think what you're describing is already present on the hud in the form of the accelerometer, that sort of tells you which way the trains is going. And let's not forget that the tutorials tell you everything about the hud, there is already a direction indicator on the hud, so people shouldn't get confused.
Speed up and slow down aren’t the correct terms to use for the power control. Unlike your depiction of the keys used with arrows left and right, the keys are not directional either. The left key is for increasing power. The right key is for decreasing power, and in trains that have a combined lever for increasing braking. If you look out of the train windscreen you will see that you are going forwards, not to the right like the gradient icon, or to the left like your keys diagram. The power keys are not directional and the gradient icon is just following signage convention, pointing to the right for forwards. The two are not connected, and shouldn’t be read as such.
