More on Interlocking

The following is an email that I received from Graham Plowman in response to my article Interlocking Turnouts and Signals. He makes some really interesting comments on how to make your interlocking even more realistic and so I obtained his permission to reprint the email here. Most of the information relates to figure six of the interlocking article so I am repeating that here to make it easier to see what is being referred to.

passing loop at a station
Hello Stephen,

A member of the british_model_railways eGroup has posted your web site to the group. Out of interest, I had a look and found a number of interesting articles.

Of particular interest to me was your article on interlocking on which I would like to make some comments and provide some additional information.

I myself am in the process of building a large OO scale layout which has fully integrated turnout and signal interlocking. The control of the layout is quite unconventional, being controlled from a PC using the 'SSI' software which my company develops and supplies. The layout has no physical control panel: it is the PC screen. SSI is a model form of the modern IECC computer control systems used by British Railways today. Via hardware connected to the PC via the serial port, the PC is able to control all turnouts, signals, isolating sections and interlocks. The actual interlocking is achieved via a scripting language built into the software, just like the real systems. Implementing interlocking in scripting makes it very easy to update. Trains are driven using conventional Gaugemaster controllers.


Of interest to me in your article was figure 6 because this is a classic situation which is often signalled incorrectly by modellers. All of what you have written here is correct, however, there is an aspect which appears to have been omitted: overlaps. In the case of fig 6, there are two overlaps. These are the section of track beyond a signal up to the point of nearest possible collision: ie the distance BB to B and BA to A.

Signalling and interlocking rules change, depending on the length of the overlaps. On the prototype railways, overlaps have prescribed lengths which are dependant on a number of factors including gradient, line speed, signalling (MAS vs Semaphore) and train braking type.

Unfortunately, when we scale prototype dimensions down to model form, we find that we are well below the minima permitted! So what I did on my layout, using prototype CAD permanent way design software (which also calculated the transitions and canting) was to use the absolute permitted minimum. It actually equates to about 17mph running on the prototype, but in model form, it is just over 2 feet long.

In the context of fig 6, what this means is that if the overlaps mentioned are not long enough, then if a train is passing MA, a train in the opposite direction is NOT permitted to enter the loop and MUST be held at MB. When the train passing MA has stopped, MA is placed at danger and MB can be cleared.

So what we have is a situation where signals are interlocked with each other on opposite ends of a layout - something which I haven't seen anyone do on a layout.

Interlocking isn't just about relating signals to turnouts. It also involves relating signals directly together, independent of turnouts. The great thing about using computer software to implement all this is that it is very easy! It is extremely difficult to hard-wire the same level of functionality!


Overlaps in fig 6 also bring on another point which most modellers nearly always get wrong. Fig 6 is a potentially a good example: trap points.

The track layout in fig 6 would only be permitted if the overlaps are sufficiently long for a train to stop. If they are not, the traps must be inserted, effectively installing crossovers where the overlap is too short ie at BA and/or BB. Where traps are installed, trains would be permitted to enter from both directions at the same time.


Another little known interlocking rule relates to bi-directional platforms.

This actually relates to my passenger loop platform as an example. Signals on each end of a platform MUST be interlocked to prevent both being pulled off at the same time. This prevents the possibility of locos on each end pulling a train apart if someone inadvertentely forgets to uncouple something.


Although what you have said about the reason for (or lack of) interlocking on models is completely true, don't forget that some modellers actually implement it for the same reasons as the prototype: to avoid collisions.

One user of our software has models which are so valuable that he cannot afford to have them collide!


Happy modelling!
Graham Plowman, Sydney, Australia
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Copyright Stephen Chapman and Graham Plowman
This letter has been reprinted with the permission of Graham Plowman