How does a coal steam engine work?
Steam at 210 psi pushes each piston with the weight of 30 tonnes, and every chuff pulls a breath of air through the fire. Take an Indian broad-gauge steam locomotive apart in 3D, from the coal on the grate to the last steam train of 1995.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- A power station on wheels. Coal burns on a grate in the firebox. Its hot gas rushes through a hundred or more fire tubes in the boiler, boiling the water around them, and out of the chimney. Steam collects in the dome, where the driver’s regulator lets it out to the cylinders.
- Coal to steam. India grades coal from G1, above 7,000 kcal/kg, down to G17. The sealed boiler runs at 210 psi, so water boils at about 200 °C, and a superheater heats the steam to about 350 °C. Fire harder and you make more steam, but a smaller share of each shovelful ends up in it.
- Steam to motion. Steam pushes on a 514 mm piston with about 300 kN, first one side, then the other. The Walschaerts valve gear sets the cut-off: early cut-off lets the trapped steam expand and saves steam at speed. The standard formula 0.85 × P × d² × s ÷ D gives about 136 kN of pull.
- The engine breathes. Used steam blasts up the chimney through the blastpipe and leaves a vacuum in the smokebox that pulls air through the fire. Work harder and the draught grows with it. Two cylinders give four chuffs per wheel turn, and the injector uses steam to push water back into the boiler.
- Safety and the fireman. Safety valves open by themselves at the pressure limit. A lead fusible plug in the firebox roof melts at 327 °C if low water uncovers it, warning the crew. Water gauge glasses show the level. These fittings must never be tampered with; India’s boiler laws make tampering with a safety valve a crime.
- Steam in India. India’s first passenger train ran from Bombay to Thane on 16 April 1853. Chittaranjan built 2,351 steam engines from 1950 to 1972, and the last regular broad-gauge steam train ran on 6 December 1995. Steam lives on in Darjeeling, the Nilgiris and at Rewari.
| Term | Meaning |
|---|---|
| Firebox | The box at the back of the boiler where coal burns on a grate, surrounded by water. |
| Fire tubes | Pipes carrying hot gas from the firebox through the boiler water to the smokebox. |
| Superheater | Pipes inside the large fire tubes that heat the steam above its boiling point. |
| Cut-off | How far along the piston’s stroke the valve stops letting steam in. |
| Valve gear | The rods and links that open and close the steam ports at the right moments. |
| Tractive effort | The pull a locomotive’s wheels give at the rail. |
| Blastpipe | The nozzle that sends used steam up the chimney, pulling air through the fire. |
| Injector | A jet pump with no moving parts that uses steam to force water into the boiler. |
| Fusible plug | A lead-cored plug in the firebox roof that melts if the water falls too low. |
Two thousand years from a spinning toy to the last broad-gauge steam train in India.
- 50 · A ball spun by steam (Hero of Alexandria, Alexandria, Egypt)
- 1712 · The atmospheric engine (Thomas Newcomen, near Dudley Castle, England)
- 1769 · A separate condenser (James Watt, Glasgow, Scotland)
- 1804 · The first railway locomotive (Richard Trevithick, Penydarren, Merthyr Tydfil, Wales)
- 1829 · Rocket wins at Rainhill (George and Robert Stephenson, Rainhill, Lancashire, England)
- 1853 · India’s first passenger train (Great Indian Peninsula Railway, Bori Bunder, Bombay, to Thane)
- 1950 · Chittaranjan’s first engine (Chittaranjan Locomotive Works, Chittaranjan, West Bengal)
- 1995 · The last regular broad-gauge steam train (Northern Railway, Ferozepur to Jalandhar, Punjab)
The full story, with 30 moments, charts, people and 56 sources: glassbox.how/e/steamengineclear/history. The data lives in history.json.
Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.
| File | What | Size |
|---|---|---|
glassbox/reel.mp4 |
Reel / Short, with captions and soundtrack | 1080×1920 |
glassbox/video.mp4 |
YouTube video, with captions and soundtrack | 1920×1080 |
glassbox/slide-1…10.jpg |
Instagram carousel | 1080×1350 |
glassbox/thumb.jpg |
YouTube thumbnail | 1280×720 |
glassbox/cover.jpg |
Share card and repo social preview | 1200×630 |
glassbox/history-reel.mp4 |
“History in 10 moments” Reel / Short | 1080×1920 |
glassbox/history-slide-*.jpg |
History carousel | 1080×1350 |
glassbox/post.json |
Post copy and schedule used by the publish kit |
This box has no accounts and no ads, and it ships its own fonts and libraries. When you run it yourself it sends nothing anywhere. On glassbox.how, the site's /bar.js also loads Glassbox's analytics: Google Analytics to count visits (it asks first in the EU, UK and Switzerland, and stays off when your browser sends Global Privacy Control or Do Not Track) and ClickTrust to detect bots.
It remembers a few things in your own browser only, and never sends them anywhere:
| Browser storage key | What it holds |
|---|---|
steamengineclear.v1 |
Which chapters you have opened, your best quiz scores, and sound on or off. |
Exactly what each one sees is at glassbox.how/privacy.
- Code: MIT. Use it, change it, ship it.
- Explanations, text, images and videos (
glassbox.json,glassbox/): CC BY 4.0. Credit “Glassbox, glassbox.how/e/steamengineclear”. - Third-party parts keep their own licences: three.js (MIT), Geist, Instrument Serif (SIL OFL 1.1).
- The Glassbox name and logo aren't covered by either licence. See the terms.
Found a mistake? Open an issue. Corrections happen in public.
It's plain HTML, CSS and JavaScript. No build step and no dependencies. Run locally, it contacts no other website.
python3 -m http.server 8000Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.
Then open http://localhost:8000.
| File | What |
|---|---|
index.html, css/app.css |
The page and its styles |
js/app.js, js/stage.js, js/ui.js, js/kit.js |
The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director |
js/steam.js |
Steam physics: the steam tables, coal grades, the boiler model, Walschaerts valve kinematics, the indicator diagram, the blastpipe draught, chart boards and particles |
js/loco.js |
One generic, unlettered coal-fired 4-6-2 locomotive and tender, with rods, valve gear, water level, fire and explode |
js/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
glassbox.json |
Title, question, explainer beats, key terms, browser storage and credits shown on glassbox.how |
reel in each chapter |
The storyboard the Glassbox studio records into short videos |
glassbox/ |
The published video, slides, thumbnail and post copy |
fonts/, vendor/three/ |
Self-hosted Geist and Instrument Serif (SIL OFL 1.1) and three.js (MIT) |