Detecting Your Trains & Separating them with Blocks
By Don Woodwell
(excerpted from Automating Your Model Train Layout, 2nd Edition)
The invention of the basic track circuit for operating railroads
in 1872 by Dr. William Robinson enables trains to automatically
operate signals by completing an electrical path between the
loco/car wheels and the rails. The train completes a battery
circuit by electrically bridging across the rails and, thereby,
activating a relay. The relay contacts then become the input
to a signaling system stating that this section of track (a "block")
What the signal system does with this information is wholly
dependent upon the railroad's operating rules. Overly simplified,
if a block is occupied, the signal aspects at either end of the
section should be “Stop” so that an engineer in an approaching
train knows that he must not enter the occupied block.
Model railroaders define detection as the ability to know the
location on a layout of a locomotive or piece of rolling stock
even when that equipment is hidden from view. For example, operating
two trains on one loop of track requires blocks so that one train
may stop before it runs into the one ahead of it. Knowing where
each train is located is the purpose of detectors both mechanical
and electrical as described in Part I.
Blocks are variable track sections whose lengths are determined
by several factors including train length, track purpose, electrical
power requirements for operations, and number of trains running
on a main or branch line. These will be covered here in Part
Throughout this article, references will be made to specific
installations and usage examples on my NC-Lines & Traction
high-rail layout in order to illustrate automatic applications
of detectors, blocks, and commercial electronic modules. The
latter – CEM for short – are any of the varied off-the-shelf
electronic modules designed and built for specific control applications
such as sound reproduction, train detection, lighting trains
with constant voltage, animation and lighting sequence timing,
signaling control, among many others. Although NC Lines is 3-rail,
the following examples also apply to 2-rail layouts. Any unique
differences will pointed out and discussed.
In the modeling world, we do the same thing with an insulated
track section (ITS) as Dr. Robinson does with his track circuits.
One of the two outside common rails on 3-rail track requires
isolation from the other in a block. When a metal wheel and axle
set bridges the two rails, it completes a circuit that may activate
signals, other operating accessories, or relays as shown.
Early forms of electrical detection relied on large relays with
sensitive coils. Power was routed to the track through the relay
coil, and when a locomotive was in the section of track wired
to the coil, the relay would activate. This system worked effectively
for the 'O' Scale and larger equipment in use in those days,
but as HO scale became popular, with its smaller motors and much
lower current draw, the series relay form of detection lost favor.
|Figure 1: Using an Insulated Track Section
An insulated track section may be used to activate a Timer circuit
for stopping at a way station.
Another early form of detection was the track switch, a mechanical
device used extensively by Lionel, American Flyer and other tinplate
|Lionel 153C Contactor Activated by a Train's Weight
It consisted of a spring-loaded contact assembly that was activated
by the weight of the passing train. Although adequate for the
simple signal and crossing gate needs of those tinplate layouts,
they were not very stable, and are not practical for a layout
where the track is fixed to the roadbed. In recent years, Lionel
and MTH in particular developed an infrared detector to replace
the mechanical devices.
|Magnetic Reed Switch Between the Rails
Magnetic reed switches are reliable detectors, but they often
require modifying locomotives or rolling stock. In practice,
when a permanent magnet mounted on a locomotive, for example,
passes over a reed switch between the rails, the magnetic attraction
momentarily pulls two metal 'reeds' together completing an electrical
circuit. The metal reeds are housed in a glass tube with leads
on each end.
One application for this detector type is stopping a train at
a station. The permanent magnet, however, must come within 1/4" or
so of the reed. This means that you must find a location on your
engine where you can mount the magnet so that it pulls on the
metal reeds. Any locomotive that is required to stop at the station
must have a magnet attached to it.
About the same time that 2-rail DC powered HO scale layouts
started to replace the larger scales, transistors were invented,
and these led to the development of true electronic detectors.
The Twin-T circuit, designed by Lynn Westcott was simple, sensitive
and effective, and it found immediate favor with modelers. The
Twin-T circuit was sensitive enough that rolling stock could
now be detected if a resistor was placed across the insulated
Detecting current flow caused by the presence of a locomotive
or lighted car within a block is enabled with modern electronic
circuits called current sensing detectors (CSD). Detection only
occurs, however, if something draws current. Rolling stock with
plastic wheels would not, however, be detected. A block must
be longer than any train that runs through it so that a lighted
caboose would be detected at the rear of the train. When it is
detected, then the block is no longer occupied.
Ideally, CSDs should be electrically isolated from track power
so that it does not take power from the track nor should it be
affected by short circuits. DALLEE Electronics accomplishes this
by having one wire to the track section physically pass through
a hole in the detection coil of its TRAK-DT device. See Figure
Current flowing in the wire due to a train's presence induces
current in the detection coil that activates a relay of the TRACK-DT.
The relay output can be applied to a signaling or other electronic
|Current Sensing Detectors--Custom Signals GCF (left) & DALLEE
ELECTRONICS TRAK-DT (right)
After Iow cost integrated circuits (ICs) -- with hundreds and
sometimes thousands of transistors on one chip -- were developed,
a number of manufacturers started utilizing this new technology
to provide sophisticated detection for model railroads. CIRCUITRON
pioneered the use of integrated circuits combined with tiny optical
sensors for optical-electronic detection (OED) in any scale.
ICs have also been used to replace the discrete transistors of
the Twin-T circuit, and they offer greater sensitivity and more
NC-Lines uses both the ITS and infrared detection (IRD) method
so as to get the most flexibility. However, there's another reason
for choosing the IRD device. If an insulated track section is
already used at the same location to activate a grade crossing
signal, you may not want both signals operating at the same time.
Therefore, you need an alternate detection means. The IRD or
OED device also may be better if you use 3-rail sectional track
with metal ties. Work can be saved since insulating a section
means bending track clips, lifting rails, sliding insulators
between the rail and the tie, and then pressing the track clips
For most 2-rail applications, the opto-electronic detectors
are the easiest to install, particularly when the track is already
in place and wired. A typical OED by CIRCUITRON is only 0.18" (4.6
mm) in diameter, and easily fit between the ties in HO scale.
As such, it is unnecessary to remove or alter any existing track,
and in most cases, no loss of performance will result even if
the Opto-Electronic detectors are covered with a thin layer of
ballast. The associated circuitry is highly sophisticated and
sensitive and will operate properly under extremely low levels
of room light. These OED circuits contain sensitivity adjustment
controls to adapt them to your existing room light.
|Opto-Electronic Rolling Stock Detector
The current-sensing type of detector is the most practical when
you desire to operate under varying lighting conditions or in
total darkness. The CSD circuits may be less costly, per block,
than the OED; however, wiring is more complicated and each piece
of rolling stock will need to be modified in order for the electronics
to detect it. Metal wheel and axle sets must be installed, and
resistors or resistance paint will have to be applied across
the axle insulator if you want to detect each piece of rolling
stock. These added costs should be considered when comparing
Installation requires an air gap in one rail at both ends of
a block. Pass the feeder wire through a hole in a component mounted
on the 1-1/4" square circuit board, solder it to the rail
between the gaps, and connect the board to 12-16 volts AC or
DC. A separate power supply isn't required. A relay energizes
when a train is detected enabling the Detective's contacts to
be used to control signals, panel lights, or other accessories.
Train detection is important for many reasons other than multiple
train operation. For example, both NC-Lines (the railroad) and
NC Traction (interurban trolleys) use detectors for signaling.
Detectors that tell where a train or trolley is located also
control signal aspects. Detector output also controls block access
so that mainline signals are coordinated with block occupancy.
Train location detectors may be used for other applications such
as activating accessories, stationary sound, grade crossing signals,
or for automatic switch and signal control.
Prototype railroads break their mainlines into blocks in order
to allow multiple trains to run safely on the same track. Block
signals inform the train crew of the upcoming blocks' status
and whether or not another train occupies them. The crew takes
action as dictated by the signals. They may, for instance, continue
at an allowable speed or reduced speed due to the blocks' status.
In real life, the train engineer can speed up or slow down his
train as necessary.
It's the purpose of the block occupancy detector to determine
if a following train must be slowed or stopped in a preceding
block to avoid a rear end collision on a subsequent block. Thus,
go-no go decisions are automatic based upon train location.
The key to stopping a following train is to place occupancy
detectors at the beginning of a block so that the following train
slows down two blocks behind the first train, and stops in the
immediately preceding block. This blocking practice should enable
the model railroad operator to maintain spacing between his trains.
Compared to prototype railroads, model trains stop in a very
short distance, therefore a block at least equal to one train
length is sufficient distance for the following train to stop.
|Mainline Space-Interval Blocks-- one possible way
to set up mainline blocks with detectors to insure space-interval
Slowing in one block or stopping in the next one ensures that
the lead train is far enough in advance to avoid a collision.
When the lead train reaches a location further along its route,
its position is detected and the following train is allowed to
resume normal speed.
Space-interval blocking such as described requires no signaling
only control of track power. Nevertheless, adding signals will
increase your layout's visual interest. Model railroads rarely
need prototypical signaling systems, but the addition of signal
components adds a tremendous amount of realism, and changing
signal lights impress viewers as trains pass by.
Track blocks also may be in yard areas or passing sidings. These
two types of blocks are static, and they are either 'on' or 'off'
as an operator controls them. Mainline blocks are dynamic, on
the other hand, if there is more than one train operating on
the mainline. The state of a dynamic block is controlled by train
A different way to run two or more trains on a mainline is to
create passing sidings each of which is a separate block. The
number of passing sidings is one less than the number of trains
running on the mainline . One or more trains can be parked in
un-powered sidings while another train runs around the mainline.
Let's suppose, for example, you want each of two trains to alternate
access to the mainline and stop in a siding. When one train stops,
the other train departs, makes one mainline run, then returns
to the siding and stops. Control of this alternating mainline
use is dependent upon a stop circuit. Power would be cut in the
passing siding block when the train's presence is detected within
the siding. After a short time delay to allow the moving train
to fully stop, power is applied to the second passing siding
block and the other train departs. Such block control is simple
to create and is a basic form of automation.
Block Indicator Lights
Indicator lights on the layout control panel activated by detectors
make it easy to spot both rolling stock and complete trains positioned
on hidden track blocks like staging tracks or sidings. On NC-Lines,
two end loops are hidden by a mountain and city, and one siding
is hidden under the mountain. Each location only requires a short
piece of insulated track section to detect the presence of a
passing train or stored rolling stock. I mounted three amber
panel Indicator lights on my control panel's track diagram corresponding
to the detector's locations. A current sensing detector could
also be used, but not an opto-electronic detector since each
track location is in the dark.
Panel Indicator Lights for Blocks
Detector circuits are extremely important for sensing train
locations and then taking some action. The action may be to control
signaling, slow or stop a train, activate grade crossing gates
and lights, or start up a rail side accessory. Blocks are sections
of track that may or may not be powered depending on train activity
on the mainline or in the yard. Both detectors and blocks are
usually part of any model railroad whether in conventional, cab,
or command control.