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mechanical-py — parametric mechanical + civil + station catalogue

Every piece of the OpenSourceRail physical catalogue — rails, sleepers, U-girders, station canopies, turnout kits, depot doors, solar mounts — is expressed as a parametric Python CAD assembly.

Design philosophy

The whole catalogue is biased toward prefabrication and bolt-together assembly. Every design choice in this package is made against three criteria, in order:

  1. Eliminate on-site structural engineering. The deployment partner's civil engineer should never have to redesign a structural member. Everything is cataloged with published load envelopes; the engineer's job is to pick a kit size, not to draw a new one.

  2. Eliminate on-site welding and wet concrete. Hot-dip galvanised bolted steel + precast concrete is the default. The only on-site concrete is pad footings. The only on-site welding is the continuous- welded rail (CWR), which is already standard practice.

  3. Ship flat, erect fast. Every assembly is dimensioned so one standard lorry (13.6 m flatbed × 2.48 m × 2.65 m height) can take a meaningful erection package. A standard station canopy is two lorry-loads of steel + roof panels; erection is 3–5 days with a small crew and a 30 t crawler crane.

What's in scope

Module Components
osr_mech.track Rail profile (54E1 / 60E1), precast mono-block sleeper, Pandrol-style fastener, assembled track panel
osr_mech.civil Precast U-girder for elevated spans (RFC 0011), at-grade/elevated ballastless slab panels, precast pad footing, precast L-unit platform edge
osr_mech.station Steel portal-frame bay, multi-bay canopy assembly, solar-roof panel, signage + lighting mast
osr_mech.rolling_stock Car body, bogie components, trainset assemblies, sensor cowl, couplers, articulation, doors, batteries, traction/electronics envelopes, T-OBS sensor pack, fit-out envelopes
osr_mech.cad_templates Fabrication templates plus supplier-neutral COTS fixture envelopes

The current CAD is an envelope and design-review package, not a homologated production drawing set. It now includes COTS-inspired fixture geometry, rolling-stock systems, and sheet-metal/chassis templates. Remaining v0.2 production-detail work is supplier-exact SKUs, weld maps, tolerance stacks, FEA-ready brackets, harness clamp locations, and controlled 2D manufacturing drawings.

Supplier-neutral fixture models intentionally remain generic until a procurement freeze. A selected SKU closes the CAD gap only when its datasheet envelope, mounting keep-outs, service-removal path, and revision are captured in the parametric source and matching drawing register.

Parametric inputs

Every component takes its parameters from the RFC-level choices the operator has already made in design.toml:

  • Rolling-stock family → platform length → canopy bay count.
  • Track-geometry preset → rail profile + sleeper spacing.
  • Station archetype → bay count + facility mast count.
  • Civil class (at-grade / elevated / bridge) → U-girder span
    • pier spacing or ballast depth.

So design.toml drives both the Rust planning pipeline and the mechanical catalogue. One source of truth for the whole deployment.

Canonical Source

The Python CAD source files under src/osr_mech/ are the design basis. Tracked FreeCAD .FCStd files under catalog/freecad/ are the compact assembly-review artifacts. Neutral CAD exports are local-only scratch files when a supplier specifically asks for them.

The source geometry uses the local osr_mech.cad facade. Under FreeCADCmd that facade emits native FreeCAD Part shapes directly; in ordinary Python it keeps lightweight volume and bounding-box metadata so the unit tests can run without a local FreeCAD install.

Key rolling-stock source entry points:

Source Scope
src/osr_mech/rolling_stock/trainset.py Trainset family assembly and motorisation
src/osr_mech/rolling_stock/car_body.py Concept-aligned layered car body: structure, exterior, interior, HVAC ducts, LV/data, HV/PV, thermal/fire routes
src/osr_mech/rolling_stock/sensor_cowl.py Large glass end cowl and T-OBS nose envelope
src/osr_mech/rolling_stock/systems.py Doors, batteries, couplers, electronics, charging, sensor packs
src/osr_mech/rolling_stock/bogie/ Powered and trailer bogie components
src/osr_mech/cad_templates/rolling_stock.py Chassis/body sheet-metal templates

Usage

# One-time: install the package in editable mode.
pip install -e .[test]

# Build a FreeCAD review assembly.
# The launcher uses native FreeCADCmd or the FreeCAD Flatpak and handles
# FreeCAD's script argument quirks.
scripts/freecad_trainset.sh --family light-metro-3car

# Build chassis/bogie and full-body FreeCAD documents with assembled
# and disassembled/exploded review states, plus geometry-check markdown.
scripts/freecad_assembly_review.sh

# Run first-pass FreeCAD/CalculiX screening FEA beam models and
# solver-result PNGs for chassis, bogie, and body load cases.
scripts/freecad_fea.sh

# Capture FreeCAD GUI screenshots from the review/FEA documents.
# Uses Xvfb automatically when available and refreshes the stable
# docs/screenshots/freecad/ latest image set.
scripts/freecad_screenshots.sh

# Build and capture FreeCAD station, ballastless track, and rolling-stock
# scene renders for at-grade, elevated, and interchange configurations.
scripts/freecad_station_scenes.sh

Testing

pytest validates volumes, masses, overall footprints, and solar-roof area against the published numbers in the RFCs. A geometry test that regresses mass outside ± 1 % fails — the RFC needs updating or the model needs fixing, but the two can't silently diverge.

Handoff to structural engineering

The tracked handoff package is the Python source, FreeCAD .FCStd review assemblies, screenshots, and design notes. A deployment partner can generate local neutral CAD exports from the same source if their toolchain requires them, but those exports are not committed because they are bulky and reproducible.

The published load envelopes (in the component docstrings) are conservative — every kit is sized for the worst of:

  • EN 1991-2 rail loading (LM71 × α = 0.83 for light metro).
  • Eurocode 8 seismic (PGA 0.3 g, which covers most of the target deployment footprint from MENA through South Asia).
  • Wind: 50 m/s basic wind speed (typhoon-class).
  • Thermal: ΔT = 70 K for steel, 35 K for concrete.

Deployment sites with harsher envelopes need a beefed-up variant; we'd rather add a "heavy" SKU than weaken the standard.

FreeCAD assembly bridge

osr_mech.freecad_trainset builds a structured FreeCAD document directly from the parametric source geometry. The resulting .FCStd file is a review and handoff assembly: parts are grouped as car bodies, bogies, onboard systems, couplers, platform interfaces, and clearance references, with placements and display colours applied for inspection.

The bridge deliberately does not make FreeCAD the source of truth. Authoritative geometry stays in the Python source; FreeCAD is the compact tracked review format for assembly inspection, drawing generation, and partner mark-up.

Licence

Apache 2.0 for the Python code. Generated CAD review artifacts under catalog/freecad/ follow the same open-hardware intent as the rest of the hardware/ tree.