Set them to the dimmer setting for a train approaching in the other direction, but
@martin2345uk put "dip" in quotation marks because it's from car/road vehicle terminology, where you're driving in the dark on an unlit road with no vehicle close in front of you or approaching in the other direction (and therefore you're using main beam/full beam/high beam headlights) but if for example (a) vehicle(s) approach(es) in the other direction, you pull the stalk to change to
dipped beam/low beam headlights ("dipping" the headlights as in lowering the angle of them / tilting them down to a less dazzling angle, even if they're actually different lights).
With LED headlights / on trains, you'd technically be dimming rather than dipping.