Railway Signalling

I’ve long been interested in railways. Not because I’m a “foamer” (UK parlance — apparently some people foam at the mouth when they get the chance to watch passenger trains move, or so the railway employees would have it) or a “railfan” (the US term — Is that supposed to be like “sportsfan”? I mean, just because I want to take a photo that has a train in it doesn’t make me a weirdo, does it? Apparently), but for the same reason that engineers tend to interested in almost everything: how does it all work?

Model of a searchlight railway signal
Not bad for a model railroad!

One part of real-world railways that is fascinating is the signalling necessary to make operations safe and efficient. It’s beguiling to an engineer in no small part because, by design, you can’t infer the behaviour of the entire system just watching the signals that go by as you’re on a train: automated signalling isn’t just about local conditions, but about the relationships between track conditions and the locations of trains across vast distances. The relevant Wikipedia pages have never been much help, either. As an unrequited model railroader, I’ve seen plenty of articles about modelling signals, and even descriptions of CTC machines and Train Orders, but still precious little about how signalling systems, as a whole, work. So I’ve long been curious.

A few days ago I came across a fantastic reference by one Carsten Lundsten about how signalling is done in North America. I’ve been engrossed. It appears the site was written somewhat for a European audience, but as far as I can tell it’s pretty informative for a Canadian and American one, too.

Rather than blathering on about which rule number a signal represents or what speed limits are, these documents concentrate on how signalling systems protect trains and how they have improved over time to provide greater automation and flexibility. If you’re interested, definitely start with Basics of North American Signaling and Safety principles.

Absolute Block Control, double track, through to Centralized Traffic Control, double track
Want to know what this means?

Be sure to make your way through to the page about Absolute Permissive Block signalling — by far and away the best explanation for APB and how APB is different than ABS I’ve ever seen, and I’ve been casually researching this for years.

Opposing signals clearning in section of main track under control of an Absolute Permissive Block signalling system

Enjoy, all ye “train geeks”.


Model photos from Richard Stallard’s site about his Marbelup Valley railway.
Signal plant diagrams from Carsten Lundsten’s site, as above.