Railroad Signalling: Introduction

Railroad Rules, Signalling, Operations

Introduction to Railroad Signalling

Mark D. Bej

[ Copyright & Licensing Agreement | Note to Contributors | Back to FAQ ]


Purpose of Signals
Principles of Signalling
Bibliography and Related Reading


  1. Please read the main disclaimer on the FAQ page.
  2. This introduction is necessarily brief and very general. If you find youself complaining about too many details on this page, the Basic Section is probably not for you. Please move on to the Advanced Section (or contribute a page or two!)


Dave Pierson, Jon Roma, and John Cooper provided helpful comments.

Purpose of Signals

One will find this statement in most treatises on signalling, in some form or other, and without intent of plagiarism, it goes something like this:

Two trains that are not in the same place at the same time cannot collide.

The same is true, of course, of any two moving bodies. The more complex the moving body, or the more expensive, or the more people it carries, or the poorer the communication is, or the larger number of such moving bodies involved, or the greater the speeds involved, or the more dangerous a collision is because of other inherent factors, the more one needs some control mechanism over those moving bodies.

Signalling thus can accomplish three objectives:

  1. Safety: prevent collisions and other mishaps; and
  2. Control: prioritize train movements, smooth operations; and
  3. Information: the providing thereof.

Thus, human beings on farms need few signals, if any, as safety is not a major issue, there are few other humans to run into, etc. Move to the city, and you find crosswalk stripes, "WALK/DON'T WALK" signs, one-way escalators [moving stairs], railings, chains near street corners (e.g., England), and so forth, either for safety or control, or both.

Cars previously required nearly no signalling. Now, with increased speeds and numbers of cars compared to 1910, cars require stop signs, lane striping (defining lanes, a control method), turn signals, different classes of roads, right-of-way rules, and lots of regulations/laws to govern their behavior.

The situation with planes, trains, and ships is analogous, and each mode has developed its own system of passing back and forth of information, i.e., signalling. The inability of trains to turn on their own produces its own unique problems which signalling helps alleviate.

Principles of Signalling, with an Example

Railroad signals are similar in purpose to highway traffic signals for cars. Keep in mind, though, in reading this, that it was highway signals that developed from railroad signals, not the other way around.

Everyone needs some warning before getting a signal to stop. For your car, that warning is the yellow light the appears for a few seconds before the red light appears. The yellow light is timed according to the prevailing speed on the road it governs and, of course, according to the required stopping distances for cars and trucks. It does not take much time or distance (relative to trains) for cars and trucks to stop. (Cars and trucks can also turn out of the way of danger.) Therefore, the yellow warning light can be provided at the same physical location (the same signal apparatus) as the red danger light.

Trains, however, are heavy. They may require 1/2 to 1+1/2 mile to stop. Thus, they need a warning well before the point where they have to stop. The arrangement must inherently be different from what is present on roads. Here are the basics of how it works.

Each section of track, from signal to signal, is one 'block'. So far as we are concerned here, the signal defines the block regardless of how the underlying hardware may work. Except in infrequent instances beyond the scope of this introduction, only one train is permitted in each block. Entry into the block is permitted or denied by the signal. That signal is said to 'govern' the block.

Blocks in the past, in the days of short trains, were often about a mile (about 1.5 km) long. With longer and heavier trains, they've been lengthened over the years, so that a more typical value on large U.S. mainlines at present is about 2-3 miles. However, there are many factors involved in determining block lengths; this will be discussed separately in the Advanced section.

A red signal can mean one of several things:

  1. The next block is occupied.
  2. If the signal is a controlled signal (under direct human control, not automatic), the person controlling it may simply want to hold the train at that location for some time, to allow some other train pass;
  3. Some other danger situation exists. This may include a switch (points) in the next block not properly lined. Or, another route may have been set which does not allow the train to proceed (e.g., at a 90° crossing).

A yellow signal indicates that the next block is clear, but that the block following that is occupied or unsafe. Thus, the signal after is red, and that the train has the length of one block to stop. A green signal, finally, means that the next two blocks (at least) are clear, and that the next signal is therefore either yellow, or also green.

It is important to note that signals do not convey certain other kinds of information such as the following:

  1. maximum authorized speed on that stretch of track;
  2. speed restrictions on curves, bridges, and the like;
  3. temporary conditions, such as temporary speed restrictions, work zones, etc.

Bibliography and Related Reading

  1. Armstrong, John. "All About Signals" [part 1]. Trains, 194?, pp. ??.
  2. Armstrong, John. "All About Signals" [part 2]. Trains, 194?, pp. ??.
  3. Above 2 articles previously available from Kalmbach Books (Wisconsin) as a monograph. Reportedly not available at present (1997).

Mark's Railroad-Related Stuff
Railroad Books of Rules and Signalling: A Home Page
Introduction to Railroad Signalling

Mark D. Bej
+1 216-444-0119