THE NUMBERS, EXPLAINED
How to use this calculator.
Rectangular conceptual tunnel envelope around a model railway. Supplied curve offsets already include the chosen rolling stock sweep; no prototype engineering certification.
Start with your inputs
Enter model rolling-stock body width (mm), body height above rail (mm), measured outward curve offset (mm), measured inward curve offset (mm), supplied lateral margin each side (mm), supplied top margin (mm). Keep amounts in the units shown on each field. The example is a starting point, not a recommended value.
| Input | Example value |
|---|---|
| Model rolling-stock body width (mm) | 40 |
| Body height above rail (mm) | 50 |
| Measured outward curve offset (mm) | 4 |
| Measured inward curve offset (mm) | 3 |
| Supplied lateral margin each side (mm) | 2 |
| Supplied top margin (mm) | 5 |
A worked example
With the example inputs above, required tunnel opening width (mm) is 51. Change the inputs to explore your own scenario.
| Result | Example answer |
|---|---|
| Required tunnel opening width (mm) | 51 |
| Required height above rail (mm) | 55 |
| Rectangular opening area (mm²) | 2,805 |
How this is calculated
Required tunnel opening width (mm)
Add body width, inward and outward sweeps, and both lateral margins.
Required height above rail (mm)
Add body height and top margin.
Rectangular opening area (mm²)
Multiply required width and height.
Symbols and units
- a
- Model rolling-stock body width (mm)
- b
- Body height above rail (mm)
- c
- Measured outward curve offset (mm)
- d
- Measured inward curve offset (mm)
- e
- Supplied lateral margin each side (mm)
- g
- Supplied top margin (mm)
Use the units shown. A list keeps the order in which you enter its values. The symbol ^ means “raised to the power of”.
Assumptions and limits
Rectangular conceptual tunnel envelope around a model railway. Supplied curve offsets already include the chosen rolling stock sweep; no prototype engineering certification.
Calculations use the values you enter. Blank or invalid inputs show an error instead of silently substituting zero. Results are displayed to useful precision; calculations retain the underlying numeric precision.
Questions about this tool
What does the result tell me?
Required tunnel opening width (mm); Required height above rail (mm); Rectangular opening area (mm²). The result applies to the method and inputs described above. Check the units and assumptions before using it in a decision.
Why might another calculator differ?
Differences in units, rounding, timing or model definitions can produce different answers. Compare the input definitions and the method, rather than only the final number. For this tool: Rectangular conceptual tunnel envelope around a model railway. Supplied curve offsets already include the chosen rolling stock sweep; no prototype engineering certification.
Are my inputs stored or sent to AI?
Calculations on this page happen in your browser. The separate AI question finder sends its submitted conversation to OpenAI; these calculator fields are not sent. Favourites and recent tool names are stored locally. A shared link contains the inputs you choose to include.
Further reading
OpenStax: Geometric formulas ↗
Background reading; this does not imply endorsement or independent review of SumFizz.
Review status
Software example and browser checks are recorded in our quality report. No external expert review is claimed. Created with coding assistance; calculations run as ordinary browser code.
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