This is quite a clever contraption. :thumb:
Surely it is a one-direction-derail. When you look at the second and third pic, it derails anything running from left to right. This way it protects the turnout out of the picture at right from any loco or car which tries to roll out of the stub, dumping it to the near side of the track.
But on the other hand it also acts as a stopping block for anything coming from the right - without derailing it. Judging from the scraped off paint it was used not too long ago.
A little more puzzling is the fact that also the derail tongue on the far track looks pretty worn. But the other, big block with the handle doesn't show any scars. So how many times has this derail really been used so far?
Did you note the padlock which prevents any fooling around? After opening and removing it you can swing the block to the inside of the track. At the same time the link rod will open the derailing tongue.
Now this would be quite a challenge to model - say in Nn3!
Ron