Probably about 30 seconds over 25MPH I would guess. From the DCS Operator's Manual.
Clickity Clack
When enabled, the sound set will automatically play the clickity-clack sounds of wheels on rails when the engine moves at speeds greater than or above 30 smph and maintains the same speed for 30 seconds or more. When you change the speed, the regular engine sounds resume until the clickity clack conditions have again been met. The speed of the sound effect varies with the train’s speed.