I posted this problem to steam discussions when the Clinchfield DLC was released, but since nothing has changed and that thread remains unnoticed, I thought I would try here. The engine audio on the CRR F7 has no doppler effect applied to it. For example, when passing a stationary camera at speed, the engine pitch stays the same throughout. It should be higher as the train approaches, and then lower as it passes. This effect is applied correctly to the bell and horn on the F7, just not the engine. Doppler effect is also present on the GP9RM that seems to share the F7's EMD 567 engine sound, and to the SD40 that came in the same DLC as the F7. I don't know why this bug exists at all, Dovetail have been using this same sound set for years.
