HO questions, answered

HO is 1:87 — the European NEM 010 standard defines it that way, on 16.5 mm track. The American NMRA gives 1:87.1. Use our scale calculator to convert sizes.

Both run on 16.5 mm track, but OO models are built to 4 mm to the foot (1:76.2) and HO to 3.5 mm to the foot (1:87). OO grew up in Britain because early motors were hard to fit inside HO-size models of Britain’s smaller trains. OO models are about 14% bigger than HO models of the same train.

The track gauge is the same 16.5 mm, and Peco sells much of its track labelled “OO/HO”. The trains are different sizes, though, so HO and OO models look wrong side by side, and accessories marked “OO/HO” vary in size.

Yes — The British 1:87 Scale Society, formed in 1994 and listed as active at Companies House. See our clubs page for other UK groups.

Märklin warns that older transformers can, in unfavourable circumstances, give too high a voltage when changing direction, that some digital models specify its newer transformers for analogue use, and that the insulation of old transformers ages in storage.

Märklin says jerky running after long storage is normal and cleaning helps; specialist dealers and Märklin’s repair service can help if it doesn’t.

Märklin’s magazine suggests a wool cloth is basically enough for the rail tops, with a little solvent such as methylated spirit for stubborn dirt; motorised cleaning wagons are also sold.

DCC is a standard for digital control: each loco’s decoder has an address so you drive locos, not sections of track. A new decoder’s short address (CV1) is 3. For more, see our sister site DCC Train Automation.

Trix says its H0 locos always have a decoder, register automatically with Märklin digital (mfx), and also run on analogue DC or DCC layouts.

A standard pocket on the ends of many European HO models that lets you pull out one coupler head and push in another. Kadee’s #17 to #20 couplers fit it; the pocket height is 8.5 mm, so check with a height gauge.

A way of dating European models to a period of railway history, from Epoch I to Epoch VI, set out in NEM 800. Makers are recommended to state the epoch so you can keep a layout consistent.

It’s the rail height in thousandths of an inch: code 100 is about 2.5 mm tall and code 83 about 2.1 mm. Lower codes look finer. Code 100 is common but represents very heavy rail. Compare codes with our rail code tool.

Not without conversion. Märklin 3-rail models pick up current from studs between the rails with a sliding shoe, while 2-rail models use insulated wheels and both rails. Märklin’s 2-rail brand is Trix.

Sectional track’s tightest curves are around 360 mm (for example Märklin C-track R1 and PIKO A-track R1). The NMRA’s RP-11 suggests larger curves for longer and more detailed models — check each model’s stated minimum radius. Our layout space helper lists the radii and works out board sizes.