H0 scale, 1:87, model railway, the different systems


Basically, there are two different main rail systems:

There are also two basic control systems; analogue and digital.
We first look at the "old fashion " analogue system.

In alphabetic order:

The AC system:
AC, for Alternating Current. This is often referred to as the tree rail system.  By many this is called "the Märklin system".
The three rails refer to the two outer rails and a centre rail or contact studs.

This system have been used by Märklin since the introduction of the H0 scale in 1935.
Some other manufacturers also offers locomotives for the three-rail AC system.

The DC system:
DC, for Direct Current.  This is often referred to as the two rail system.
The positive voltage is applied to one track, the negative voltage in the other track.

Some more details:

AC system:
The driving current is applied to a centre rail, or contact points, both tracks are connected to ground.

From Wikipedia

Driving speed:
The driving speed is altered by adjusting the applied voltage from the controller or transformer, between approx 4 and 16 VAC.  All the locomotives on the track travels at approximately the same speed.

Driving direction:
The driving direction is changed by applying a 24 Volt pulse to the locomotive, activating a reversing unit in the locomotive.
This means that the driving direction is controlled in each locomotive.

The DC system:
The two rails are insulated from each other.  Plus is applied to one track, minus to the other track.

Driving speed:
The speed is altered by adjusting the applied voltage from the controller or transformer.  All the locomotives on the track travels at approximately the same speed and in the same direction.

Driving direction:
The driving direction is changed by reversing the polarity of the tracks.  All locomotives on the track travel in the same direction.

the above information about speed and direction refers to the "old fashion" analogue systems.
Most manufacturers offers digital control systems, where the power supply are different kinds of digital pulses, which combines power supply and digital information.
The track system and the locomotive layout remains the same

Analogue operation:

AC system:
The power is controlled by an adjustable transformer, applying the requested AC current to the three rails.
Approx 4 to 16 Volts for driving, approx 24 Volts for direction change.
All the locomotives on a track will run at approx. the same speed, but the direction is depending on the electro-mechanically changeover unit (reversing unit) in the locomotives.

DC system:
The power and polarity is controlled by an adjustable transformer, applying the requested DC voltage and polarity to the two tracks.  All the locomotives on a track will run in the same direction an approx. the same speed.

Analogue AC locomotives from other manufacturers than Märklin often have a small electronic device, not the normal reversing unit found in Märklin locomotives.

This is also the fact for a few Märklin locomotives with DC motors, as this Märklin 3118.  They are not digital compatible.


Digital operation:
Basically, the system could be the same, regardless of two-rail or three-rail operation.
But, the different manufacturers have their own systems, that do not comply. They all have different communication protocols.
Digital operation requires a digital train controller and a decoder in each locomotive.  The controller and the decoders must have the same protocol to be able to communicate.

Locomotive layout:

AC system:
All the wheels are connected to the locomotive chassis, the wheel axels are not insulated (short circuit, if placed on a DC track).  A slider, also called pick-up shoe; is installed under the locomotive, between the wheels.  The slider is insulated from the rest of the locomotive.

DC system:
The two wheels on each axel are electrically insulated, as the two wheels are running on a rails with different electric polarity.  The wheels on one side of the locomotive are electrically connected, normally via a contact plate touching the inside of each wheel.  So are the wheels on the other side of the locomotive.
The contact plates are connected to the brushes on the electric motor (with permanent magnets) or eventually to the decoder.

A Märklin AC locomotive:

All the wheels are connected to the chassis, as ground, the slider (or pick-up shoe) picks up the current from the centre contact points.

This is a drawing found on internet, once made by Märklin, of a traditional analogue locomotive with an electro mechanical reversing unt

1 center rail (studs) 5 brushes    9 reversing unit magnet 13 rocker contact
2 pick-up shoe (slider) 6 motor field magnet 10 reversing unit armature 14 light bulb
3 FM choke 7 rotor 11 reversing unit return spring 15 locomotive chassis
4 main solder point 8 rotor shaft 12 reversing unit slider  

During normal operation, 4 to 16 Volt AC, the current flows from the slider to the main solder point, to the lamp, reversing unit magnet and to the front motor brush.
The lamp glows with an intensity according to the applied voltage.
The reversing unit magnet creates a magnetic field, but the return spring is stronger than the magnetic field; the slider does not move and the rocker stays in it's position.
The front motor brush applies a voltage to the rotor.  The current flows through the rotor, to the rear brush and continues to the lower part of the double motor field magnet.  From the field magnet to the rocker to ground (chassis)

When applying the short 24 Volt AC pulse for direction change, the reversing magnet becomes stronger than the return spring.  The reversing unit armature moves towards the magnet and the slider flips the rocker contact to the opposite position, thus changing the current flow from the one half of the motor magnetic field to the other.
This way the motor magnetic field flips 180 degrees (north becomes south and vice versa) while the current flow through the rotor remains the same.  The turning direction of the AC motor changes.

An analogue Märklin locomotive:

In this case the locomotive includes a small electronic unit controlling the front lights and the reversing unit.

Digital locomtives:
In a digital locomotive the reversing unit is replaced by a digitally operated decoder, which converts the power supply to the correct motor voltage and light operation, controlled by digital pulses constantly applied to the track.
driving speed and direction is controlled in the locomotive.

There are many different types of decoders, see my pages about digital conversions.