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Short Definition

Control based on magnitude of voltage applied to the track.

The locomotive is controlled by the magnitude of the voltage applied to the track. Also referred to as Direct Current or DC.

Unlike Digital Command Control, which always has the same voltage applied to the track, analog uses control of the voltage to regulate speed and direction. You control the track, not the train.

Originally, the trains ran on alternating current, or AC, the same form of power supplied by the utility to your house. A simple transformer and rheostat provided control. At early as 1934 there were discussions about converting your layout to direct current. The technology of the time meant Flat DC using car batteries (automotive electrical systems were 6 volts until the mid-50s.) The reason for using a battery was the cost of electronics. A vacuum tube power supply was large and costly, and solid state selenium rectifiers were costly as well. The practical method was a battery and a battery charger. Another possibility was the rotary converter, but they were very expensive. They were employed by prototype railroads to convert 60Hz AC to 25Hz used by electric locomotives. (That was another issue, there was no large scale electrical grid in North America until the 1950s, and the line frequency depended on whose equipment your utility purchased.)

Direct current began to gain traction with the introduction of HO scale as the motors were smaller. Solid state electronics made possible smaller and more reliable power packs supplying direct current. Another factor favouring direct current by the 1950s was the standardization of the power system in North America to 60Hz, simplifying the design of power supplies. (Transformers work best and most efficiently at the frequency they were designed for.)

Analog Control

Once Upon a Time, this is how you would control your trains.

Analog control is accomplished by varying the voltage supplied, from zero to twelve volts, and direction is controlled by reversing the polarity of the voltage supply.

These are defined in the NMRA Electrical Standard S-9.

From S-9

This STANDARD establishes ELECTRICAL requirements for interchange and safe and satisfactory performance of two rail equipment on model railroad layouts.

Two important points from the Standard are:


A. Full throttle voltage available at railhead shall not be less than 12 volts direct current at maximum anticipated load. (1)
B. High frequency current superimposed upon the rails shall not interfere with the normal operation of Powered Equipment. (2)


A. Direction control by polarity reversing shall be provided. Positive potential applied to the right hand rail shall produce forward motion. (3)
B. Speed control means by voltage/current reduction shall be provided.
While the standard states 12VDC, this is the minimum acceptable voltage under load, and can be as high as 16VDC. Many magazines will use 12VDC to determine the top speed of a locomotive which makes comparisons easier.


(1) When using a power source delivering a wave with greater harmonic content than full wave rectified sine wave, exercise care not to operate in such a manner to exceed the rated current or otherwise overheat the motor.

(2) The least restrictive wiring requirements of the U.S. National Electrical Code call for no combination of voltages in layout wiring to exceed 49 volts. Modelers in other countries are encouraged to check with local authorities.

(3) The term "right hand rail" as used herein means the rail to the right of the observer standing between the rails with their back to the front of the locomotive.


North American Utilities used all manner of line frequencies, and voltages, until they standardized on 120/240VAC 60Hz for domestic service. Prior to that, it could be any frequency from 16 to 400Hz. Westinghouse (of Air Brake fame) installed 25Hz alternators at Niagara Falls, because that worked best with the turbines, making 25Hz very common in that region. Direct current service in New York City was terminated in a few years into the 21st century. The last 25Hz alternators at Niagara Falls (Sir Adam Beck GS) were removed from service in 2011.
Twenty five Hertz is used with large commutator type motors in industrial service, which don't like frequencies like 60Hz. The Pennsylvania Railroad employed 25Hz AC on their electrified sections, which drove the GG1s down the track for many years. Today, the power is still 25Hz. Power was generated by line side stations, or purchased from the local utility. If needed, large rotary converters were used to generate 25Hz power if it was not available from the utility at that frequency. Today, that conversion can be done with solid state frequency converters.