Interlocking is the glue that physically prevents a signalman from being able to signal two or more trains onto the same piece of line at the same time. For many years the mechanical interlocking was provided by a series of sliding bars called tappets interacting with locking bars, the latter fitted with a series of locks.

Locking of this sort is still used at surviving mechanical lever signal boxes on our national railway network and on railway networks in many parts of the world. Heritage railways, in particular, continue to use tappet interlocking and will be doing so for many years to come.

Despite this and the fact that the principles involved are the foundation on which nearly all signalling past, present and future depends they remain something of a mystery to many people.

This paper, written by one of our members, Reg Instone, describes the history and principles of the tappet locking systems used in one form or another by so many railway companies and signalling equipment manufacturers. Modellers who wish to replicate prototype signalling and use it correctly on their layouts also need an understanding of interlocking and this is an excellent primer in the subject.

This signalling paper is supplied as a photocopied booklet. Copies are created to order - we don't keep piles of them on the shelves - so there can easily be a three week gap between you ordering and the booklet dropping through your letter box. The wait will be worth it!

Need this on a different media? Contact us via the link at the bottom of the page and we'll see what we can do.