GAUGE & SCALE


Choice of gauge and scale are always constraints for modellers. As well as available domestic or workshop space, plus financial and personal preference, modellers of Indian railways also have to make choices about their track gauge and scale. In particular, they have to decide how accurate they wish their track gauges to be, in relation to the chosen scale.

There are two strategies for deciding this: pick a scale or pick a gauge. To some extent, the choice will depend on how much commercial support each modeller needs. However, if more than one gauge is contemplated (BG plus MG or NG, for example), a reasonably accurate scale/gauge ratio for each gauge is essential for correct appearance.

Choosing an existing scale has the advantage that components, especially fittings, will be available. Choosing an established gauge will mean that wheels and mechanisms, in particular, can be utilised. The following two tables show the possible combinations.

Gauge & Scale -- Choosing scale by Gauge
Usual Scale for Standard Gauge (SG) Scale For Indian
TG (mm) Scale mm/ft Ratio BG MG NG (2ft 6in) NG (2 ft)
/ft /ft /ft /ft
6.5 Z 1.3855 1:220 1.1818 1.9812 2.6000 3.2500
9 N 1.9050 1:160 1.6364 2.7432 3.6000 4.5000
12 TT 2.5000 1:121.92 2.1818 3.6576 4.8000 6.0000
16.5 HO 3.5000 1:87 3.0000 5.0292 6.6000 8.2500
18.2 EM 4.0000 1:76.2 3.3091 5.5474 7.2800 9.1000
18.83 P4 4.0000 1:76.2 3.4236 5.7394 7.5320 9.4150
22.23 S 4.7625 1:64 4.0418 6.7757 8.8920 11.1150
32 O 7.0000 1:43.5429 5.8182 9.7536 12.8000 16.0000
45 1 10.0000 1:30.48 8.0357 13.7160 18.0000 22.5000
65 G 13.6701 1:22.2968 11.6071 19.8120 26.0000 32.5000


By picking an existing gauge, you MAY be able to use available track. The Pune Railway Society recently produced a WDM2 diesel model to 1:50 scale, nearly correct for 32mm gauge track. Hence it could use O gauge flexible track, etc. But the sleeper spacing and rail sizes could be a problem: 3mm scale on 16.5, eg, looks promising until one realises that Code 70 rail is the equivalent of 7" high in 3mm (1:101.6) scale: 132 lb rail. For authenticity, modellers would be confined to Code 55 HO track, the equivalent of a 90lb FB rail at 5½" high (such track is commercially available). Even then, sleeper spacings would probably be dubious in the smaller scale.

In the larger scales, 16mm scale models on "O" gauge track, representing 2ft gauge, 2ft 3in and 2ft 6in gauge prototypes, are quite well established.

At the least, therefore, existing gauge involves some compromises. The alternative is to pick a scale, and then work out the necessary gauge(s).



TRACK GAUGE for Given Scale
Designation Scale

(mm/ft)

 Normal

Gauge

 Ratio

Scale

 Correct ScaleTrack Gauge (mm)
SG BG MG 2ft 6in 2ft
Z 1.3855 6.5 220.0000 6.52 7.62 4.55 3.46 2.77
N 1.9050 9 160.0000 8.97 10.48 6.25 4.76 3.81
2mm Fine 2.0000 9.42 152.4000 9.42 11.00 6.56 5.00 4.00
Brutish N 2.0595 9 148.0000 9.70 11.33 6.76 5.15 4.12
TT 2.5000 12 121.9200 11.77 13.75 8.20 6.25 5.00
Brutish TT 3.0000 12 101.6000 14.12 16.50 9.84 7.50 6.00
3mm Fine 3.0000 14.2 101.6000 14.12 16.50 9.84 7.50 6.00
1:100 3.0480 100.0000 14.35 16.76 10.00 7.62 6.10
HO 3.5000 16.5 87.0857 16.48 19.25 11.48 8.75 7.00
P4 4.0000 18.83 76.2000 18.83 22.00 13.12 10.00 8.00
American OO 4.0000 19.00 76.2000 18.83 22.00 13.12 10.00 8.00
1:75 4.0640 75.0000 19.13 22.35 13.33 10.16 8.13
S (3/16") 4.7625 22.23 64.0000 22.42 26.19 15.62 11.91 9.52
1:50 6.0960 50.0000 28.70 33.53 20.00 15.24 12.19
O (1/4") 6.3500 1-1/4" (31.75mm) 48.0000 29.90 34.92 20.83 15.88 12.70
O 7.0000 32 43.5429 32.96 38.50 22.97 17.50 14.00
1 (3/8") 9.5250 1-3/4" (44.45mm) 32.0000 44.85 52.39 31.25 23.81 19.05
1 10.0000 45 30.4800 47.08 55.00 32.81 25.00 20.00
1:25 13.1233 25.0000 61.79 72.18 43.06 32.81 26.25
G 13.6701 65 22.2968 64.36 75.19 44.85 34.18 27.34



Of the various gauges, and scales at which they might be equivalent, remarkably few are even within 1/4 mm of the correct gauge. There is only ONE exact match -- 5' 6" gauge in 3mm scale, at 16.5mm.


In the smaller scales, and given the limited space available in Indian (and many other!) houses, 3mm scale may have considerable advantages. While there is an exact match only with 16.5mm (HO) gauge, it is clear that MG track at 9mm (HOe/HOn2½), and NG at 6.5mm (Z) are not wildly inaccurate. In each case, the use of existing RTR or kit mechanisms, plus wheelsets. track, and other materials, could be contemplated. (It would be interesting and useful if a table showing wheel diameters and spacings of commercial models could be complied, with apprpriate data for Indian locos that could be modelled.)
HO workers using available track could use 18.83mm (P4) or 19mm (American OO) track for BG, 12mm (HOm) for MG, 9mm (HOe) for 2ft 6in gauge, and 6.5mm (Z) for 2ft gauge.
4mm scale offers 22.23mm (S) for broad gauge (at least one modeller is using this combination), 12mm (HOm again!) for MG, 10.5mm (HOn3) for 2ft 6in gauge, and 9mm gauge (HOe) for 2ft gauge.
However, if you are keen to mix gauges, the best option may be to opt for hand-built track to the correct gauges. Nowadays a wide range of useful components is available, especially in 4mm and 3.5mm scales. Making or procuring suitable gauges is likely to be the biggest hurdle, but even this is not insuperable. Accurate track gauges may involve you in modifying mechanisms, but this is not always as hard as it seems, and additionally, many high-quality parts are nowadays available for the scratch-builder.

WHEEL and TRACK STANDARDS

All railways, models included, depend on five critical dimensions to govern the running of wheels on the rails, and critically, through pointwork. The Track Gauge (TG), wheel Back-to-Back gauge (BB), the Crossing Flangeway gauge (CF), the Check Gauge (CG), and the Effective Flange (EF) must all be consistent with each other. Ideally, all vehicles on a given layout should have identical critical dimensions. Some latitude can be tolerated, especially in the "coarse" standards, but, broadly speaking, the more latitude, the greater the probability of derailment.
Most RTR modellers simply accept the wheel standards offered by the manufacturers of the equipment they use. However, this, while seemingly an easy option, can have pitfalls. Many manufacturers make wheels and track to their own standards, and these are not necessarily compatible. In this, U.S. modellers have a great advantage, since nearly all manufacturers for that market conform at least nominally with the NMRA standards. British and European modellers, however, will find significant variation in standards, and may have to engineer solutions. This can be as hard as, or harder than, scratchbuilding.
Generally, there are at least three standards options: "coarse", "finer but not fine" (FNF -- my own designation), and fine scale. Most coarse scale workers accept what the manufacturers give them, sometimes limiting their purchases to one or two makers so as to restrict incompatibilities. They are represented by the ordinary RTR buyer in Z, N, TT, HO, (English) OO, O, and G gauges. FNF modellers generally insist on a uniform wheel and track standard, often not the coarsest of those on offer from the Trade. Typical examples are HO modellers using the NMRA RP25-88 standard, EM gauge (in Britain -- 4mm scale on 18.2mm gauge track) and "O fine" practitioners. Fine scale standards, once regarded as impractical curiosities, are catching on rapidly worldwide. They now exist for Z scale, 2mm scale, 3mm scale, HO (the P87 standards), 4mm scale (P4 and S4), and O gauge (Scaleseven and 1/4"AAR).
To find out more about standards, you can refer to the NMRA's website, and to the Scalefour Society and P87 SIG sites. There are numerous others, too, and any basic book on model railways should include an explanation of gauge and scale. For a discussion of basic track-wheel relationships in models, it is very hard to go past the "Protofour" series which appeared in the "Model Railway Constructor" in 1967.

BACK TO MAIN MODELLING PAGE