These are some pictures of a demo unit I made in 3mm MDF to develop the idea for the door ctaches and interlocks on the basis a model makes it easier to both develop and explain.

The one thing the model doesn't represent well is the mass the real thing would have. In particular the arm being just two layers of MDF is a bit light. I would envisage the real thing being cast or profile cut brass.

There is some discussion at the end on options etc
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FRONT VIEWS FIRST

Starting position. Doors open with the hook raised

A stop to the rear prevents the hook going further back but it is open and retracted clear of the doorway

By going over centre its weight keeps it in the open position

  • The switch sensing the arm position is open

  • The switch sensing the door position is open.

  • The OK green LED at the top is unlit
  With the hook dropped it is prevented from falling further by the peg at the left hand end bearing on the hooks extension.The stop is important for two reasons.
  1. it prevent the arm on the back that works the hook position switch form taking any more force than is needed to close it and
  2. It keeps the hook aligned so that if the doors were closed the hook would ride over the peg on the opposite door and latch

In this view

  • Hook switch is closed
  • Door Switch is open
  • OK green LED is unlit
FG

In this view the doors are closed but the hook is still raised.

  • The Hook Switch is open

  • The door switch is closed

  • The OK LED remains unlit
 

Lastly the doors are closed and the hook is latching them closed on the outside of the cars and will remain latched until next opened by an operator

  • The hook switch is closed

  • The door switch it closed.

  • The OK LED is lit.

 
 

Now from the rear.

The doors are open and the door switch is away from the striker plate of the opposite door.

The hook is out of shot but is raised and the arm it operates at the rear can be seen against the stop. The hook switch, out of shot, is open.

  • Hook switch is open
  • Door switch is open
  • OK LED is unlit
 

Now the doors are closed but the hook is still raised

  • The door switch is closed

  • The hook switch is open

  • The OK LED is unlit

 
 

Now the doors are closed and the hook is lowered and in the locked position.

  • The door switch is closed

  • The hook switch is closed

  • The OK LED is lit
     
   
 

Originally the hook was the profile shown above. I then modified the left hand end when I realised that limiting the lift worked fine but left the hook in the doorway just right to catch the punters

You can also see the right hand end has a but of a mod with some glued on stick to make it ride up over the peg on the opposite door. The shape of the finished item would need to sweep up in a more elegant set of curves - I have an example in a photo but I can't find it at the moment.

The door position switch would be different on the real thing. The switch would be mounted in the door void and the upright would have a sprung loaded plunger through it the pressed the microswitch. It would be brass tube and plate with a spring in the tube. It would have all the physical stops built in so the switch took no load

 		  
  The hook would need to be a more curvey shap like the one in the picture if you get as far as looking at the hook


The picture is from a series of New Zealand Railways safe working posters