what do that look like and what they do?
Here's a video : The WD is the Blinking white light
Lo - V Entering Nevins Street Layup from Atlantic AveThey are placed at switch points and Yards/layups . They measure the trains speed and bring it to a halt if you exceed that speed.
Here's a good definition from Subchat:
"If active, the first light attached to the WD sign will be solid, which is a warning to the T/O that you're entering an active WD area.
A bit further up will be a sign specifying the speed for the area, like "WD-20". The light beyond this sign, which was previously off, will start to flash as a warning that it's currently calculating your speed. If you're doing more than 20 MPH, it will continue flashing, and eventually trip your train. If you're maintaining 20 MPH or less, the flashing will soon turn solid, to let you know you're in the clear for now." If you speed up before the "WD End" sign , they you're train will be tripped , making it more harsh then a Timer signal.
I'll try and post a video of it in action in openbve later