Railway signals play a critical role in the movement of trains on a line. Generally signals convey information about the occupancy of the track ahead and whether the train can safely proceed or needs to stop. Unlike with cars, the stopping distance for a train is generally greater than the sight distance of the engineer, so the safe operation of a train is absolutely dependent on the signal system telling the engineer what to expect ahead. For this article we will stick to signals common to North American railroads, and mainly focus on signals that can be found on modern Class I railroads.

Definitions

Before we can get into the basics of signaling there are some terms that need to be defined so that the concepts will be easier to explain:

• Aspect: What the signal shows (e.g. green, yellow, red, green over red, etc.).
• Block: A stretch of track into which the entry of a train is controlled by a signal or other form of authority.
• Diverging route: The route through the curved side of a switch (known in the industry as a turnout).
• Indication: What the aspect means.
• Interlocking: A group of one or more switches and associated signals.
• Maximum Allowable Speed (MAS): The maximum speed at which a train is allowed to operate though an area, as defined in the employee timetable.
• Name: What the aspect is called (clear, approach, stop, etc.).

Highball!

Nearly everyone in the railroading world knows the term “highball”; it’s synonymous with “full speed ahead”, and basically gives the engineer authority to start moving or continue moving. Conductors and dispatchers often use it when communicating with the engineer of a train. But where does it come from?

Quite literally, a high ball.

One of the earliest forms of railway signal used to control the movement of trains was the ball signal, which consisted of a red ball on a rope and pulley, mounted on a pole. When the ball was raised (high), it meant the next block was clear and the approaching train could proceed at MAS. So let’s continue at MAS into railway signaling!

Signaling Basics

In North America, there are two general categories of signaling used by Class I Railroads: speed signaling and route signaling. The types basically do exactly what it says on the cover: speed signaling tells the engineer how fast to go, and route signaling tells the enginer what route the train is lined for. Route signaling is the more common type of signaling, with speed signaling being used in dense corridors where there are multiple routes a train could take. In these complex areas using speed signaling the engineer may not know exactly where their train will be going, but they know what speed they need to be going to get there safely. With route signaling, the engineer knows exactly where their train will be going, and has to remember (or look up in the timetable) what is the maximum speed they can be going to get there safely.

Signals are placed at two distinct locations on the railroad: block signals are placed at the beginning of a block (in any direction that a train is allowed to travel within the block) and home signals are placed at interlockings. The block signal preceeding a home signal is known as a distant signal.

Basic block signals consist of a single head with three lights: red, yellow, and green. On most North American railroads, each signal head can only have one light lit at a time and thus can display three aspects: stop, approach, and clear, respectively. There is also a fourth color that is used occasionally: lunar white (aka lunar), which displays an aspect known as restricting. Here is a quick explanation of what these four aspects mean:

• Clear (green): The block is clear, and the next signal does not indicate stop; proceed at MAS.
• Approach (yellow): The block is clear and the next signal indicates stop; prepare to stop at the next signal.
• Stop (red): The block is occupied, do not proceed.
• Restricting (lunar): The block is occupied, proceed at restricted speed. Restricted speed is defined as a speed at which you can stop within half of your sight distance, maximum 15 mph, expecting to encounter a train. (Why half sight distance? There may be another train coming the other way at restricting speed!)

Home and distant signals need more than one signal head to display a sufficient amount of information, and thus may have as many as three heads. The aspects displayed by these three heads is about the status of interlockings, so for now we will just stick with the basic four above. The aspects and indications are listed again below with the aspects for two and three head signals:

• Clear (green / green over red / green over red over red): The block is clear, and the next signal does not indicate stop; proceed at MAS.
• Approach (yellow / yellow over red / yellow over red over red): The block is clear and the next signal indicates stop; prepare to stop at the next signal.
• Stop (red / red over red / red over red over red): The block is occupied, do not proceed.
• Restricting (lunar / lunar over red / lunar over red over red / red over lunar / red over lunar over red / red over red over lunar): The block is occupied, proceed at restricted speed.

Types of Signals

Common types of signals in use on North American railroads include color lights, searchlights, position lights (PLs), color position lights (CPLs), and position color lights (PCLs). Color lights are by far the most common, and the aspects in the basics section above are those of a color light signal.

Color Lights

Color light signals are by far the most common type of signal in the United States at present. Many railroads have begun to replace other types of signals with color lights in an effort to standardize the signals across their entire system. Most color light signal heads consist of three lenses, with green at top, yellow at middle, and red at bottom. Sometimes the signal heads are oriented sideways, though this is not common. Another type is the “Tri-Light”, which features the three colors arranged in a triangular shape on a circular background. A proper tri-light would be very difficult to recreate at scale with Lego, given the triangular arrangement on a circular background.

Many designs of color light signal have been built in Lego, and have even been included in Lego sets, such as 4541. One major gripe that I personally have with these is that they are “go/no go” signals that lack a yellow approach indication. While these types of signals are often used in yards, terminals, or other slow-speed locations, they would practicaly never be used on the mainline (at least not in North America). A train would not be able to stop for a red signal if it was operating at full speed when the engineer saw the signal, which is why the approach indication is so important: it tells the engineer the next signal is red, so be prepared to stop then. This gives them plenty of time to slow the train, before they even see the red signal.

Searchlights

Searchlight signals consist of one light with an internal mechanism that moves different colored lenses in front of the light. Working searchlights are also difficult in Lego, as they would depend on a multi-color LED.

Position Lights (PLs)

Position lights were developed by the Pennsylvania Railroad, and consist circular heads with lines of lights at different angles (the center light is common to all aspects). These position of these lights mimic the position of the blade of a semaphore signal, the predecessor to these signals. Most PLs have a second head with at least one light, so they can shown additional speed aspects.

Here’s an example of a Lego position light signal:

Position lights were installed throughout the Pennsy’s territory, and can still be found along the Northeast Corridor and the lines of many other railroads, like CSX, Norfolk Southern (NS), and the Long Island Rail Road (LIRR, sic). Variations include dwarf and pedestal PLs, which are used in yards, terminals, and other areas where a full PL is not needed. (Note that “dwarf signal” is the correct industry term for these small signals used in yards, terminals, and other restricted clearance areas.)

Amtrak’s Northeast Corridor between New York City and Washington DC was once the Pennsylvania Railroad mainline, and thus was chock full of PLs. At some point Amtrak added colored lenses to their PLs, making them Position Color Lights (not to be confused with Color Position Lights, which we’ll talk about next). PCLs also resemble “Red Eyes” PLs, which are standard PLs with two red lights for the stop indication, but red eyes PLs have a center lamp for use with the approach and clear indications.

Color Position Lights (CPLs)

Color position lights were developed by the Baltimore & Ohio Railroad around the same time as PLs. They feature one main head with up to four indications (the ones listed above), and as many as six marker lights above and below the main head to display speed information. CPLs can be differentiated from PLs because there is no middle light and the aspects are in color: vertical is green (clear), horizontal is red (stop), 45° right is yellow (approach), and 45° left is lunar (restricting).

Here’s an example of a Lego CPL Lego:

Similar to PLs, CPLs have dwarf varitions as well:

Additional Complexity

This covers the basics of railway signaling in North America. Getting further into route and speed signaling, home signals, and interlockings will require their own articles, as these concepts get very complex. The indications of signals vary slightly from railroad to railroad, thus adding to the complexity. I was told once that Amtrak engineers on some routes need to be able to interpret as many as 6 different types of signals by memory, as their routes use the tracks of various freight railroads and thus their signal systems too.

Here’s one fun shot I wanted to share, of a cable car signal in San Francisco. I took this shot to tease my CEE 410 Railway Signaling professor by asking what the indication is for “red X over green over green arrow over white person?” He just laughed and suggested we order another beer!