What this reconstruction is—and is not.
A runnable, full-scale SI teaching model, with pressure-derived motion and a closed-linkage Walschaerts mechanism. It is not a factory-drawing reconstruction or a validated locomotive-performance predictor. Missing original pivot, casting and port measurements prevent a claim of exact Er791-81 geometry or timing.
Repair release 1.1 / What changed
The audited joint, sealing and passage defects have been addressed in the source: separately stacked coupling bearings, fitted rod eyes, a genuinely slotted expansion link and die, forked crossheads, grooved piston rings and rod packing, bored axleboxes, released brake clearance, open valve windows, hollow steam conduits, perforated tubeplates and open draught outlets. Cylinder force and volume now account for piston-rod area. Downstream steam markers use local cylinder flux after regulator closure.
Validation scope: automated checks cover these specific reconstructed fits, sampled motion configurations and passage openings. They do not certify every solid intersection, stress, leakage, original valve timing, or prototype dimensions. The shared repair dimensions are marked E in the assembly ledger, data/assembly.mjs.
1 / The production record
The Russian/Soviet Э (E) family is identified by the official Patriot Park museum as the world’s most numerous steam-locomotive family, with more than 11,000 built during 1912–1957. Technical histories report somewhat different totals depending on which imported subclasses, modifications and production records are included. This is a family-level record: it includes successive E, Eg/Esh, Eu, Em and Er production groups, not 11,000 identical Er locomotives. The Cyrillic Э family is not the American-built Russian Е (Ye) 2-10-0 family.
2 / One reference configuration
Эр791-81, built in 1955 in Poznań, works number 2216. The selected configuration is late Polish-built Er: five coupled axles, twin simple-expansion cylinders, Walschaerts gear, extended firebox, 40 large superheater flues, shared external dome/sandbox casing, inclined front steps, enlarged cab skylight and a high P27 four-axle tender. Polish Er production ran 1949–1957. The specific preserved cutaway locomotive is the visual reference, not a license to mix earlier E-family details into the model.
The scene represents original 1,524 mm operating gauge, rather than its 1,435 mm delivery-wheel arrangement or the modern rounded 1,520 mm railway standard. Original service colours are interpreted; the museum’s coloured cutaway paint is not presented as its operating livery. The unit roster identifies its tender as works no. 01828. The model uses a P27 type reconstruction; that identification does not supply measured tender geometry.
3 / Dimension and evidence ledger
S — source-backed: explicitly reported in the cited technical literature or institutional account. P — provisional: reported in a secondary compilation but not corroborated against an original drawing. E — estimated: chosen for a mechanically coherent reconstruction. None is a new physical survey of Er791-81.
4 / What actually drives the model
Operate mode: firing heats a lumped water/steel reservoir. Saturation pressure is evaluated from the IAPWS saturation relation. A pressure-dependent regulator orifice feeds a finite steam chest. Geometric valve openings govern admission and exhaust through compressible-flow orifices. Each of four cylinder chambers conserves mass and integrates internal energy, including compression/expansion work. Piston force is (rear pressure − atmosphere) × (bore area − rod area) − (front pressure − atmosphere) × bore area. Rear swept volume uses the same annular area; effective valve-orifice area is capped by the reconstructed passage cross-section. Wheel torque is force × the exact slider-crank displacement derivative. Both cylinders contribute to acceleration against rolling resistance, quadratic drag and a brake-force proxy.
Wheel speed is not prescribed in Operate mode. With the regulator closed, an initially stationary unpressurised chest cannot supply sustained motive work. Residual steam can continue to do work after closing the regulator. The rail odometer and tyre rotation share the same angular integral, assuming no slip.
Inspect-cycle mode: an explicit external motor turns the mechanism at 6 rpm, in the selected direction, while the same chamber solver continues to run. The solver computes the external torque that motor must supply. The locomotive does not translate. Angle stepping switches to this mode and advances the fluid model over the corresponding time interval. It is not falsely described as pressure-derived operation.
5 / Linkage, timing and state overlays
The slider-crank is solved analytically. An eccentric-rod circle intersection closes the return-crank/expansion-link chain. A constrained solution closes the curved die slot, fixed radius rod, fixed hanger, combination lever, crosshead drop link and union link. Rigid rod meshes are translated and rotated, never stretched to chase disconnected targets. All five drivers on each side share one phase; the two sides differ by 90°.
The reverser changes the lifting arm, die position and actual geometric cutoff. Displayed cutoff is measured from the resulting admission closure, not equated with slider percentage. Valve lands and port openings use the same lap values as the fluid solver. Front and rear cutoff differ because of the reconstructed geometry; they are not claimed to reproduce factory timing.
Water, hot-gas, admission, expansion, exhaust and compression colours are educational overlays, not literal fluid colours. Flow markers appear only when the corresponding modeled flux exists; their spacing and size do not represent particles, density or exact velocity. Gas passages remain separate from boiler water. The inset section uses the current piston and valve positions; its vertical spacing is schematic.
6 / Disclosed simplifications and remaining gaps
The boiler is a lumped saturation reservoir, not a resolved two-phase pressure vessel. The superheater has an estimated, bounded outlet-temperature law, not a conjugate heat-transfer solution. Steam is an ideal gas in cylinders and chest; no condensation, leakage, real-steam equation of state, detailed draft coupling, resolved exhaust-receiver dynamics, water hammer or valve bounce is solved. Regulator area, discharge coefficients, heat losses, clearances, moving inertia and resistance are estimates.
No wheel slip, suspension dynamics, rail curves, coupler slack, train-consist forces, individual brake-rigging dynamics, realistic injector entrainment or lubricator/air-pump machinery is implemented. Tender inventory is modeled, but coal shape is not continuously excavated. The firebed, flame shape, auxiliary pipes, stays, platework and cab controls are representative rather than drawing-exact. Tubeplates now have continuous webs with open bores. Hollow superheater plenums, element connections and cored-passage approximations are still reconstructed, not authentic casting sections. Water height uses an affine inventory-to-height proxy shared with the crown-cover validity check; it is not a geometric volume inversion.
The reduced model stops beyond 70 km/h or below its minimum boiler-water threshold. These are educational validity limits, not original locomotive safety specifications. Numerical mass-flow and temperature guards are counted and exposed. X-ray and cutaway are visual tools; they do not remove physical mass or change chamber connectivity. Cutaway removes the viewer-side exterior/casings; the adjacent section diagram clarifies passages which are difficult to read in 3D.
Acceptance boundary: the software demonstrates a complete, connected heat-to-motion chain and constraint-preserving mechanical relationships. Because original Er791-81 geometry, valve port drawings and settings were not recovered, it does not establish the requested drawing-verified, dimensionally exact reconstruction. Use it as a documented engineering demonstrator and a foundation for replacement with authoritative measurements.
7 / Sources and reference access
- Patriot Park — Er788-49 museum record. Institutional evidence for the E-family production record and family production period. Its exhibit is not the selected 1955 locomotive.
- L. L. Makarov, Паровозы серии Э (2004), technical history in online transcription. Late Er changes, pressure, cylinder and driver dimensions, grate, 40-flue development and P27 capacities. The printed engineering drawings were not independently inspected.
- RailGallery — Er791-81 roster. Individual locomotive identity, manufacturer, 1955 date and works number 2216. Secondary registry.
- Harveyqs — cutaway Er791-81, Russian Railway Museum, 12 November 2017. Primary photograph, CC BY-SA 4.0. Used as a visual reference; image bytes are not bundled or redistributed.
- Russian Er technical compilation. Provisional dimensional leads and 109-small-tube count; lower confidence than an original drawing. These values remain marked P.
- Don Ashton — Walschaerts gear. General engineering topology and geometric considerations, not an Er factory setting sheet.
- IAPWS — Supplementary Release on Saturation Properties of Ordinary Water Substance (1992). Saturation-pressure correlation. Only this correlation, not a full steam-property package, is used.
8 / Reproducibility
This exhibit runs the Er Steam Lab source: the immutable SI specification, geometric solver, thermodynamic model, native WebGL 2 renderer and procedural assets, together with its automated tests. The simulation loads no runtime libraries or remote model files. CSV exports contain actual simulation samples, not prerecorded traces. Keyboard shortcuts act while the 3D view is focused: Space pause/resume, X X-ray, C cutaway, D dimensions, period to step 5°, arrow keys to orbit, Escape to release the view.
Er Steam Lab 1.1 © 2026 Er Steam Lab contributors, released under the MIT License. This exhibit is a port of that release; the narrated guide and page integration are DigiSim additions.
