bunker

A bunker is the compartment that holds coal on a steam locomotive. On tank engines, it is integral to the locomotive frame itself, positioned behind the boiler and cab. On tender engines, the coal supply rides in a separate trailing vehicle, but that trailing coal bin is also called a bunker or coal bunker. The size varies widely: small shunting tank engines might carry 2 to 3 tons, while main line locomotives routinely carry 8 to 12 tons or more.

The design of a bunker is shaped by the need to keep coal accessible to the firebox while protecting it from weather and vibration. Most bunkers taper downward toward a hopper section equipped with delivery chutes that lead coal toward the footplate where the fireman shovels it into the firebox. The coal must move freely, so the bottom is sloped and sometimes fitted with mechanical devices such as shakers or pushers to prevent bridging, a condition where coal arches over the hopper outlet and blocks flow.

Tank engine bunkers sit above and behind the firebox, often mounted high on the boiler casing to allow gravity feed. Their shape is constrained by axle limits and platform gauges; a bunker too large makes the engine top-heavy or overwide for certain routes. Tender bunkers are larger and more boxy since they sit on a separate frame with its own axles. Water capacity is always a separate concern: some tenders combined a bunker with a water tank in a single unit, while others kept them in distinct sections.

Coal quality and grain size affect bunker design. Friable coal, especially from certain collieries, breaks into dust and slack in the bunker, creating handling friction and blocking. Hard coal from other sources stays lumpy but flows less readily. Firemen complained when coal changed source or quality mid-shift; a bunker optimized for one type would frustrate work with another. Ash and clinker fallout from combustion also accumulates underneath the bunker, requiring periodic cleaning.

The term 'bunker' entered rail vocabulary early in steam locomotive history and remains standard in British and Commonwealth railways. American railways more often say 'coal box' for the tank engine variant. The word likely derives from bunking or stacking, referring to the stored mass itself rather than its container. By contrast, a 'tender' refers to the whole trailing vehicle, while 'bunker' or 'coal bunker' specifies the fuel compartment only.