I had originally planned to start writing about the construction of my layout but I am going to jump ahead a little and talk about sensors, a subject that has been the focus of much of my recent work on my layout. I'll get back to layout construction in subsequent posts.

The basic purpose of a sensor, in the context of model railroading, is to detect whether or not there is a train on a particular piece of track. Now, there are some new kinds of sensors that can additionally identify which specific train. However, they have not gained widespread popularity at the current time. In any case, as I will explain later in this series of postings, its rarely necessary to be able to identify individual trains. You can generally get by just fine with a simple yes/no status from the sensor. When a train is detected by a sensor, there are two ways to use the information generated by the sensor:

Send a notification message to a computer: There are a number of model railroad specific programs that can handle such notifications including JMRI, Railroad & Co. Train Controller, RocRail and Win-Digipet.
Directly control some electronic gadget such as signals, grade crossing gates, lights, sound effects. In addition, some DCC turnout controllers can be configured to directly select routes based on sensor notifications.

The main model railroad applications for sensors are listed below:

CTC (Centralized Traffic Control) installations require sensors to help the dispatcher (or line controller) determine whether a particular block of track is occupied by a train. Track circuits are a commonly used kind of sensor on prototype railroads that implement CTC. A number of model railroaders have implemented CTC on their layouts in a very sophisticated manner.
Computer control of the layout: If you wish to have a computer running trains on your layout, you will need sensors to allow the computer to keep track of the location of trains accurately. In fact, you will need a lot of them. My friend Dale Schultz has described this approach to automation very nicely nicely on his web site. As I have indicated previously on this blog, my own interests lie squarely in this direction.

Control automatic block signals: Many prototype railroads use automatic block signal systems on sections with heavy traffic. Among the model manufacturers Atlas offers a line of automatic block signals.
Specific layout function control: Even those model railroaders who prefer manual of their trains, often use sensors for a number of specific functions such as:

Simplifying the operation of staging areas, especially hidden ones. It is useful for operators to be able to quickly determine which staging tracks are occupied at the moment.
Operating grade crossings: Trigger flashing of lights, wig wags or crossing arms when a train approaches a grade (i.e. level) crossing.

Types of sensors

Broadly speaking, there are two kinds of sensors (note that this is my own terminology. I have not seen this used anywhere else but I think it serves a useful purpose so please bear with me):

Point sensors: This class of sensors are used to detect the presence of a train at a particular point in the layout. If you are interested in layout automation, the use of point sensors is, more or less, mandatory since the computer needs to know exactly where a train has reached. However, they can also be used for some of the other applications listed above. A number of technologies have been used to implement point sensors including infrared, reed switches, mechanically trigged switches and (recently) RFID.
Span sensors: This class of sensors are used to detect the presence of a train anywhere on a section of track. Span sensors are a natural fit for CTC and automatic block signals. The main technology for implementing span sensors is by detecting current drawn by the electric motors in locomotives and from special resistor wheel sets in other rolling stock. In the case of three rail systems like Märklin, it is also possible to take advantage of the fact that wheels are electrically conductive and create a span sensor by electrically isolating one of the outer rails in a block.