Prototype Information
On prototype railroads all track curvature is functional. They are introduced only as necessary as a means of allowing trains to move from place to place in the most economical manner.
Prototype curves are designed not only to change direction but to factor in prototype usage. Prototype usage depends on train weight and desired speed through the curve, and required elevation (banking) run out. As a result, prototype curves are preceded by easements, and superelevation in design.
Modeling Information
Railroad modeling track can be made to change direction in one of three ways: the simple curve, the compound curve and the reverse curve.
1) The simple curve is used when the direction of change is small, such as a minor change of direction on a main line, or at a turnout. The circumference of a circle is the simplest curve because it is the easiest drawn or laid out, and because its form is, at any point, the same. Any straight line which touches the circumference of the circle without cutting it, however far the straight line may be produced, is called a tangent.
2) The compound curve change directions by a gradually varying rate instead of a uniform rate. These gradual changes are known by their engineering terms: "spirals," "transition" curves, "easement", "tapering" curves, etc. They all mean about the same thing, its the engineering techniques that set them apart.
3) The reverse curve
is one formed by two curves turning in opposite directions and meeting
tangent to each other. This curve is primarily found at turnouts.
Degree of Curvature.
Prototype curves are measured by degree of curve. A Degree of Curve is
defined as: The angle subtended by a 100-foot chord. This comes from
the practice of surveying right of ways along the line of the route by
offsets. Example: A curve changes direction 15 degrees over a 100 foot
chord. The curve is a 15 degree curve. See 0.4.5.1 Degrees of Curvature Table for additional information.
The
Shafer Brothers Logging Company was supposed to have some very sharp 30
degree (193 foot radius) curves. The Denver & Rio Grande
narrow gauge line had 20 degree (288 foot radius) curves. Most standard
gauge railroads have/had one to ten degree curves.
Easements.
AREA 10 Chord Spiral, was the North American recommended practice adopted early
in the 1900's. The design specifies length along spiral (track line) and
degree of curve. There are 16 computations necessary to draw a chord
spiral. It confused me and I'll leave it to the Railroad Engineeers to
figure it out. See 0.4.5.2 Easements for Curves for additional information.
Superelevation. On
the prototype, curves are superelevated, or banked, to allow faster
speeds through the curve and to reduce lateral forces, which could
overturn the train. You can see this superelevation at NASCAR races, the
outer part of the track is raised on the curve.See 0.4.5.3 Superelevation For Curves for additional information.
Modeling Information
In the book Creative Model Railroad Design, John Armstrong discusses layout design's use of curves in railroad modeling. There are only two purposes for curves in railroad modeling: functional and cosmetic.
- The functional curve is one the design must have to make the trains run where we want them to go - i.e., that is the 90 degree curve in the corner of the layout room, or the 180 degree turn back curve at the end of a peninsula.
- The cosmetic curve is one introduced in the design to represent the appearance of the prototype.
Functional curves
- The simple curve is used when either operations do not require the engineering complications of easements or superelevation. The radius of the curve is identified (in inches). See 4.2.4.1 Laying out a Simple Curve for additional information.
- The compound curve is used when easements and/or super-elevation is wanted or needed for railroad modeling operations. See 4.2.4.2 Laying out a Compound Curve for additional information.
- The turn back curve is used at the end of peninsula benchwork where the track has to make a complete 180 degree turn in direction.
- The turnout curve is identified as either a substitution or closure radius (in inches).
- A simple reverse curve (called an S-curve in layout design) is where the diverging turnout track reverses direction to then run parallel to the main track (sidings or spurs). You also see this reverse curve at cross-overs. See 4.2.4.3 Laying out a Simple Reverse Curve for additional information.
|
Simple curve xxx |
Compound curve xxx |
Turnback curve xxx |
|
Turnout curve xxx |
Simple reverse curve xxx |
The cosmetic curve.
When it comes to representing the appearance of the prototype there are at least four that come to mind to me that railroad modelers have had an interest in and duplicated on their railroad model:
- The Pennsylvania Railroad's Horseshoe Curve,
- the ATSF Tehachipi Tennessee
- Northern Pacific's switchback crossing of Stampede Pass before the construction of the tunnel at Martin, Washington.
- The Milwaukee Roads crossing of St. Paul's Pass, the Bitter Root Mountains near Avery, ida.
The myriad of reverse curves is one formed by two curves turning in opposite directions and meeting tangent to each other. Think of the railroad right of way as a curvy snake. See 4.2.4.4 Laying out a Cosmetic Reverse curve.
References
Reference material includes railroad
engineering documents, plans, five Railway Engineering & Maintenance
volumes from 1929 to 1955, and more.
Walter Mason Camp, Notes on Track, google books Chapter V.
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