HO glossary

The usual system outside the Märklin world: the two rails carry the current, speed is set by the voltage and direction by the polarity.

Märklin’s system: both outer rails are a common return and current is picked up from a centre conductor — on today’s Märklin track a row of studs between the rails. 3-rail and 2-rail stock are not interchangeable without conversion.

Märklin’s clip-together 3-rail track with a moulded roadbed and room underneath for turnout motors, decoders and wiring. It replaced M-track in 2001; Trix makes a 2-rail version.

A coupling on a sliding mechanism (“Kinematik”) that keeps vehicles close together on the straight and lets them move apart on curves.

A setting stored in a DCC decoder. CV1 is the short address, 1–127, and is 3 when the decoder leaves the factory.

A standard way of controlling trains digitally, defined by the NMRA’s S-9 standards. Each loco has a decoder with its own address, so you control locos rather than sections of track; products that conform work together whoever makes them.

The small electronic circuit in a digital loco that reads commands from the track. Standard plugs include 6/8-pin, 21MTC and PluX.

Finer-scale British 4 mm gauges that replace OO’s 16.5 mm track: EM uses 18.2 mm and P4 (Scalefour) 18.83 mm.

The European way of dating a model to a period of railway history, from I to VI, set by NEM 800. Sub-periods use a small letter, such as Epoch IVb, and dates differ by country.

The North American equivalent of epochs. There’s no formal standard; makers such as Athearn group models by date bands.

The distance between the rails. HO standard-gauge track is 16.5 mm between the rails.

Narrow-gauge railways modelled at HO scale (1:87): H0e runs on 9 mm track and H0m (metre gauge) on 12 mm track.

Model railway scale of 1:87 on 16.5 mm track (the NMRA in America gives 1:87.1). In Germany and other continental countries it is written H0 — H and the number zero — because it is half the size of gauge 0. See the scale calculator.

The North American style of coupling; Kadee makes the original Magne-Matic type, including versions that fit NEM 362 pockets (#17–#20).

The size limit for trains on a real railway. Britain’s is smaller than Europe’s and North America’s, which is why early British models were enlarged to 4 mm scale (OO) while keeping HO’s 16.5 mm track.

Other Märklin 3-rail track systems: M-track (replaced by C-track in 2001) and K-track, which has no moulded roadbed. A transition piece joins M-track to C-track.

Märklin’s newer digital format: mfx decoders register themselves with the controller, sending the loco’s name and functions. mfx decoders also understand the older Motorola format.

The tightest curve a model is designed to run through. The NMRA’s RP-11 groups HO models into classes from 14 in (356 mm) upwards; makers state a figure for each model.

Märklin’s digital system, introduced in 1985 and based on the Motorola format.

European model railway standards (Normen Europäischer Modellbahnen), published by MOROP — for example NEM 010 for scales and NEM 362 for coupler pockets.

The standard coupler pocket that lets you swap one coupler head for another. HO and S use the same pocket; its height above the rail is 8.5 mm.

The National Model Railroad Association, the American body that publishes standards (S) and recommended practices (RP), including the DCC standards. It has a British Region.

The main British scale: 4 mm to the foot (1:76.2) running on the same 16.5 mm track as HO. OO models are about 14% bigger than HO models of the same train, so they look wrong side by side.

The height of the rail in thousandths of an inch: code 100 is 0.100 in (about 2.5 mm) tall, code 83 about 2.1 mm, code 75 about 1.9 mm. Compare them with the rail code tool.

A feedback channel that lets a DCC decoder send information back to the command station.

Märklin’s older coupler; its current close-coupler head can be combined with it.

How much smaller the model is than the real thing, written as a ratio. HO is 1:87, so 87 mm on the real railway is 1 mm on the model.

Peco’s sectional OO/HO track (code 100) with fixed curves; its No.1 radius curve is 371 mm.

A special piece that joins two different track systems or rail codes — for example Märklin M-track to C-track, or Peco code 75 to code 100.

Roco’s digital control system.