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 Design of Tomix and Kato N-Gauge Sectional Track Systems


Design of Tomix and Kato
N-Gauge Sectional Track Systems

by R. D. Kerr

Did you ever wonder why a model railway straight track piece is the length that it is, or why a curved track piece has the radius that is has? Or why some sectional track systems seem to have better layout design capabilities than others?

This page presents the Tomix N-gauge Fine Track system in the context of basic sectional track system design considerations. It offers a direct comparison to similar N-gauge Kato Unitrack pieces, which is actively exported and internationally better known. It will explain why the variously sized pieces are offered, how they work together, and how they are designed to meet particular needs. The tables below include just those track pieces that essentially define the overall systems.

Tomix produces two related and compatible track systems: its basic railway system and the "Mini Rail" system, which offers tight-radius curves and points/turnouts. Both will be presented here.

In general, Tomix offers far more points/turnout sizes, larger-radius double viaduct curves, the tighter "Mini" curves and turnouts, Wide PC Track, and Wide Tram (with the compatible Moving Bus roadway system). Kato offers more single-track viaduct pieces (of limited value), double-track curves (all super-elevated), tighter "Compact" curves and turnouts, and the Unitram street trackage system. Although they compete, each manufacturer goes its own way in a number of areas.

I will use the metric system dimensions used by the designers themselves, since I find them the easiest to use. For those people used to the "English system" who do not have a sense of metric size, remember that 280 mm equals about 11 inches, 140 mm is about 5.5 inches, and 37 mm is about 1.5 inches. (Note that 1.5 inches is also the standard track spacing for N-Trak modules.) The drawings below were made with AnyRail software, which includes Tomix track libraries.

Guide to Decoding the Tomix Track Designations (Kato is similar)

S = Straight, xxx = mm length, (F) = Fine Track
C = Curved, xxx = mm radius, -xx degrees of arc, (F) = Fine Track
X = Crossing, R or L = Right or Left crossing, xxx = mm length, -xx = degrees of crossing angle, (F) = Fine Track
P = Points (turnout), R L Y or C = Right or Left departure curve, Y-shaped, or Curve-on-curve, xxx = mm radius, -xx = degrees of arc, (F) = Fine Track
Prefixes: D = Double track; H = elevated viaduct track; N = Neo (points/turnout wiring)
Suffixes: SL = Slab track; WP = Wide with Precast sleepers; WT = Wide Tram; (F) = Fine Track


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Basic Sectional Track System Parameters

The basic parameters of a sectional track system seem pretty obvious. They include the length of a standard straight track piece, the radius and arc of a standard curved track, and the standard spacing between parallel tracks.
Basic Track Parameters Tomix Fine Track Kato Unitrack
Regular Track
Standard Straight S280 S248
Standard Curve C280-45
C280-15
R282-45
R282-15
Standard Spacing 37 mm 33 mm
"Mini" Track (Kato's equivalent is called "Compact")
Standard Curve C140-60
C140-30
R150-45
Standard Spacing 37 mm 33 mm
Wide Tram Track (paved street)
Standard Straight S140-WT not made (Unitram instead)
Standard Curve C140-60-WT
C140-30-WT
not made (Unitram instead)


Fractional / Multiple Straight Track Sizes

These straight track sizes are basic fractions or multiples of the standard straight. (Other odd sizes are also produced and are discussed later.)
Fraction/Multiple of Standard Tomix Fine Track Kato Unitrack
Regular Track
1/4-length S70 S62
1/2-length S140 S124
3/4-length use S70 + S140 S186
Wide Tram Track (paved street)
1/2-length S70-WT not made (Unitram instead)


Incremental Curved Track Sizes

These curved track sizes are greater or lesser radius than the standard curve by one or more increments of track spacing. (Other odd sizes are also produced and are discussed later.)
Multiples of Track Spacing Tomix Fine Track Kato Unitrack
Regular Track
+3 C391-45
C391-15
R381-30
+2 C354-45
C354-15
R348-45
R348-30
+1 C317-45
C317-15
R315-45
R315-15
-1 C243-45
C243-15
R249-45
R249-15
-2 not made R216-45
R216-15
"Mini" Track (Kato's equivalent is called "Compact")
+1 C177-60
C177-30
R183-45
-1 C103-60
C103-30
R117-45
Wide Tram Track
+1 Wide Tram C177-60-WT
C177-30-WT
not made (Unitram instead)
-1 Wide Tram C103-60-WT
C103-30-WT
not made (Unitram instead)


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Crossings and Related Short Straight Track Sizes

Once you get past basic straight and curved track pieces, the design issues start to get more complicated. The size of crossings is influenced by the standard straight track length and the standard track spacing. The angle of crossings produced is related to curve arcs produced. Some short odd-sized straight pieces are produced to match one or more crossings to standard straight lengths. (Coincidentally, they also serve well to fill odd gaps in sectional layouts.)
Crossing or Short Straight Tomix Fine Track Kato Unitrack
Regular Track
15-Degree Crossing Right XR140-15 X15R (186 mm)
15-Degree Crossing Left XL140-15 X15L (186 mm)
Note: The Kato 15-degree crossovers do not comply with the Kato standard 33mm track spacing design; however, they create a 49.5 mm spacing (1.5 standard).
30-Degree Crossing X72.5-30 not made
Note: This crossing is meant to be used with both tracks at 15 degrees, such as in the center of a double-crossover made from four points/turnouts. To understand its 72.5 mm length see "Other Straight Track Sizes" below.
90-Degree Crossing X37-90 X90 (33 mm)
Straight (= 1/2 of Track Spacing) S18.5 (x 2 = 37) not made
Straight (= 1/4 Straight - Spacing) S33 (37 + 33 = 70) S29 (33 + 29 = 62)
Straight (= 1/4 Straight - 1/2 Spacing) not made S45.5 (33 / 2 + 45.5 = 62)
Wide Tram Track
90-Degree Crossing X37-90-WT not made (Unitram instead)
Straight (47.5 + 37 + 18.5 = 103) S47.5 not made
Straight (= Track Spacing) S37 not made
Straight (= 1/2 of Track Spacing) S18.5 not made


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Other Straight Track Sizes

The trigonometry of curved track arcs and straight track lengths dictate other straight track lengths, for use at angles other than 0 degrees and 90 degrees.
A straight placed at 45 degrees that is the square root of two (1.414) multiplied by a regular straight track length will project a length (at 0 degrees) and a transverse spacing (at 90 degrees) both equal to the regular length (equilateral right triangle).
A straight placed at 30 degrees will project a transverse spacing of half (0.5) of its length and will project a length at 0 degrees that is the square root of three (1.73) multiplied by that spacing (30-60-90 degree triangle).
General formula: A straight of length X placed at Y degrees will project a transverse spacing equal to X multiplied by the sine of Y degrees, and will project a length at 0 degrees that is X times the cosine of Y degrees. For example, the Tomix S72.5 straight listed below, when placed at a 15 degree angle, has a transverse spacing of 72.5 * sin (15) = 72.5 * 0.259 = about 18.5 mm (half the Tomix 37 mm standard track spacing). It has a projected length of 72.5 * cos(15) = 72.5 * 0.966 = 70 mm (1/4 of the Tomix 280 mm standard straight track length).
Straight (Length Basis) Tomix Fine Track Kato Unitrack
Straight (projects 1/4 Straight
at 45 degrees)
S99 not made
Straight (projects 1/4 Straight
at 15 degrees)
S72.5 S64
Straight (= 1/2 Standard Straight
+ 1/2 Standard Spacing)
S158.5 not made


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Turnouts (Points) and Related Track

In a similar manner to the table above, there is a relationship between the standard track spacing and the curve, arc and length dimensions of turnouts (points). A turnout, along with a complementary curve to make a siding parallel to the straight track, needs to be sized to locate the siding at the standard track spacing distance from the main track. Unfortunately, for space reasons, the standard curve radius that is chosen typically does not work this way. The only exception is the Tomix Mini Points, where the 140 mm radius 30-degree arc creates the standard track spacing of 37 mm. This lets you substitute a turnout into any 140 mm radius curve. Since Mini Rail was added to the Tomix N track product line well after the line was first designed and produced, this is either well planned or very lucky!
The regular Tomix (and Kato) system therefore has a basic turnout with an odd curve radius that is not the standard radius, and that does not even fit into the incremental curve radius system. Therefore these turnouts create a need for a matching odd-sized curve track to be added to the product line (and in the case of Kato N Unitrack, additional odd straight pieces and joiners). Please note that the Kato turnouts are technically not true #4 and #6 turnout designs, but these names are commonly used to identify them.
Turnouts & Related Track Tomix Fine Track Kato Unitrack
Standard Turnout
Standard Turnout Right (manual) PR541-15 not made
Standard Turnout Right (electric) PR541-15 EP481-15R (#4)
Standard Turnout Left (manual) PL541-15 not made
Standard Turnout Left (electric) PL541-15 EP481-15L (#4)
Curved Track to Match Turnout C541-15 R481-15
Straight to Support Turnout not needed S60R (tapered roadbed)
S60L (tapered roadbed)
Note: Kato R481-15 turnouts require use of one of the two S60 straights, plus a special regular or insulated UniJoiner (all come included with the turnout).
Additional Turnouts
Smaller Turnout Right (electric) PR280-30 not made
Smaller Turnout Left (electric) PL280-30 not made
Note: These small Tomix turnouts are 140mm long (1/2 standard length) and match the 280 mm standard radius. They can therefore be easily subsituted into layout designs.
Larger Turnout Right (electric) not made EP718-15R (#6)
Larger Turnout Left (electric) not made EP718-15L (#6)
Curved Track to Match Turnout not needed R718-15
Note: The larger Kato turnouts, like the Kato 15-degree crossings, do not comply with the Kato standard 33 mm track spacing design; however, they create a 49.5 mm spacing, which equals 1.5 times the standard spacing.
Wye ("Y") Turnout (electric) PY280-15 not made
Double Crossover (electric) PX280 WX310
Note: The Kato double crossover shares frogs and moving point rails with the R781-15 turnouts, but unlike those turnouts it complies with the Kato standard 33 mm track spacing.
Double-Slip Switch Right (electric) PXR140-15 not made
Double-Slip Switch Left (electric) PXL140-15 not made
Curve-on-Curve Turnout Right (electric) CPR317/280-45 not made
Curve-on-Curve Turnout Left (electric) CPL317/280-45 not made
3-Way Turnout (electric) PRL541/280-15 not made
3-Way Turnout (electric) PLR541/280-15 not made
Note: Curve-on-curve turnouts permit longer clear straight sections for station platforms or storage sidings and yards. Right and left turnouts can be combined to make a curved crossover. These turnouts maintain the Tomix 37 mm standard track spacing.
"Mini" Turnout
Std. Turnout Right (manual) PR140-30 not made
Std. Turnout Right (electric) PR140-30 EP150-45L
Standard Turnout Left (manual) PR140-30 not made
Standard Turnout Left (electric) PL140-30 EP150-45L (#4)
Curved Track to Match Turnout not needed not needed
Straight to Support Turnout not needed not needed
Note: These "Mini" Tomix turnouts are 140mm long (1/2 standard length) and match the 140 mm "Mini" standard radius. They can therefore be easily subsituted into layout designs, as can the Kato "Compact" turnouts.


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Multiples of Standard Track Spacing

Tomix (and Kato) station platforms, pedestrian overpass bridges, control towers, maintenance buildings, grade crossings and so on are designed to work with the standard track spacing or a multiple of it. Often the spacing must be 1.5 times the standard (55.5 mm for Tomix and 49.5 mm for Kato N) or 2 times the standard spacing (74 mm for Tomix and 66 mm for Kato N). These spacings can be made using track pieces available in the track systems.

Other Track Sizes

The curve listed below does not fit in the incremental radius pattern, does not relate to a turnout, and has an odd curve arc. It is intended to be used as two pieces in an "S" shape on one of two straight parallel tracks. In this situation it widens the track spacing by 18.5 mm (half the standard track spacing) over a length of 210 mm (3/4 of the standard straight length). See drawing above. It can also be used as a large-radius curve, similar to the Kato R718-15 curve.
Other TrackTomix Fine TrackKato Unitrack
605 mm Radius 10 Degree Curve C605-10not made
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