Mechanical railway signal box interlocking model

This is a model of the interlocking system used in some mechanical railway signal boxes (particularly in the UK, where are a number of these signal boxes are still controlling modern trains). In a mechanical signal box, each point or signal has a lever which can be pulled to operate it, and then there is a mechanical interlocking which stops you setting up conflicting routes. Basically, the interlocking stops you “doing anything stupid” - and it is all implemented mechnically with notches and slots in pieces of metal.

This model reproduces a simple three-lever example of this interlocking. Specifically, this is a tappet interlocking along the British Railways (1943) design. I have excluded the levers themselves, and so you just have the tappet bars (in green in the pictures) to pull.

This model is based on a converging railway junction as shown in the diagram below:

In this diagram, the labels 2 and 3 (corresponding to levers 2 and 3) are for signals, and label 4 is for a set of points. All trains travel from left to right - so all trains end up on the right-hand side of the diagram, but they may start on the top or bottom track on the left-hand side. In this case, the interlocking must:

• Ensure that signals 2 and 3 are not cleared at the same time (as two trains may then collide at the points)
• Ensure that if signal 2 is cleared then the points must be set for straight ahead ("normal")
• Ensure that if signal 3 is cleared then the points must be set for the other route ("reversed")

To clear a signal, pull the green tappet towards you - and similarly, to change a point from ‘normal’ to ‘reversed’, pull it towards you.

To print and assemble:

• The print profile is set up for print ‘by object’, so we can have multiple colour objects on one plate without wasting filament. There are two plates, one for the base and one for the other parts
• Assemble using the picture for guidance: all of the green tappets need to go in the right way around (the numbers should all face the same way), and then the two locking bars can be inserted - again, the correct way around (look at the pictures)
• Look at the diagram above, or print out the PDF copy in the Documentation section below

To operate:

• Just pull the levers towards you to clear signals (levers 2 and 3) or set the points to reverse (lever 4)
• You will find that some levers are locked by the interlocking, depending on the state of the other levers. For example, if lever 3 is pulled then lever 2 can't be pulled, and lever 4 needs to be pulled to be able to pull lever 3. Try and find all the combinations that work!