A steel box lifted off a ship, dropped onto a rail wagon and pulled away by a truck three days later without anyone opening the doors that is the whole idea in one sentence. An intermodal container is built to standard dimensions with standard lifting points, so it moves across sea, rail and road while the cargo inside stays untouched. That single design decision reshaped freight. It is why a pair of doors sealed in one country opens, still sealed, in another.
The Standard Is the Actual Product
Nothing about the box is accidental. ISO 668 fixes the external dimensions and maximum gross weight; ISO 1161 fixes the corner castings, the eight cast steel fittings that cranes, twistlocks and lashing gear all grip. Because every quay crane, chassis and rail wagon expects the same fitting in the same position, a unit built in one country is handled without modification anywhere else. Ports invested in equipment once and could then serve every carrier. Shippers stopped designing packaging around a specific vessel. The steel is ordinary. The agreement on where the corners sit is what changed freight.
What Makes a Movement Intermodal
The term describes the journey, not the hardware. A movement qualifies when cargo travels by two or more modes – sea, rail, road, sometimes barge while staying inside the same unit the whole way. Nothing is unpacked and repacked at the transfer points. Compare that with break-bulk, where drums are lifted piece by piece from a truck into a hold and out again at the far end. Every one of those touches costs labour, costs time and carries a share of the damage risk. Making the unit itself the thing that gets handled removes them.
The Sizes You Will Actually Encounter
Three formats cover the overwhelming majority of traffic. The 20-foot unit runs 6.058 m long and holds roughly 33 cubic metres. The 40-foot doubles the length to 12.192 m for about 67 cubic metres. The 40-foot high cube adds a foot of internal height, taking capacity near 76 cubic metres, and now dominates on routes carrying light, bulky goods. Both standard lengths are typically rated to a 30,480 kg maximum gross, so the 20-foot fills on weight long before it fills on space while the 40-foot runs out of room first. Terminal and vessel capacity is quoted in TEU, with a 40-foot counting as two.
Reading the Markings
The codes stencilled on the side are not decoration. Under ISO 6346, every unit carries a four-letter owner code ending in U, a six-digit serial and a check digit, which together identify it anywhere in the world. Beside them sits a size and type code describing the format. The CSC safety approval plate, required under the international convention on safe containers, records the approval reference and the date of the next examination a box with an expired plate should not be moving loaded freight. The plate also gives tare weight, maximum gross and permitted payload, which is the figure a loading team needs before anything goes in.
Where the Box Goes Beyond the Port
Plenty of these units never see saltwater again. They serve as secure site storage on construction projects, as workshop and equipment stores, as temporary offices after modification, and as the structural shell for modular accommodation. For general dry cargo and for most static storage, a standard GP container is what people reach for, since it is cheapest to source, easiest to replace and simplest to resell when the project ends. Specialised formats exist for what the standard box cannot take: reefers for temperature control, open tops for crane loading, flat racks for oversized items, tanks for bulk liquids.
How a Loaded Unit Actually Travels
A typical export leg starts at the shipper yard, where the box is stuffed, the doors closed and a numbered bolt seal fitted. That seal number goes onto the bill of lading and becomes the reference for whether anyone opened the doors in transit. A haulier moves it to the terminal, where a straddle carrier or gantry lifts it into the stack and later onto the vessel, locked to its neighbours by twistlocks. At discharge it transfers to rail or road for the inland leg. The consignee sees the same seal number they were quoted, or raises an exception before unloading anything.
Condition Grades and What They Permit
Second-hand stock sells against recognised grades. One-trip units have carried a single cargo from the factory and look close to new. Cargo-worthy units have been surveyed as fit for international shipping and hold valid CSC certification. Wind and watertight units keep weather out and suit storage but are not certified for sea freight. As-is stock is sold with known faults and priced accordingly. The grade determines what you can legally do with the box, so buying storage grade is a mistake if there is any prospect you will later want to ship it.
Before You Commit
Inspect in person, or insist on dated photographs of the roof, door gaskets, floor and lower rails, since corrosion starts wherever water sits. Check the doors open and close without a pry bar and that the locking rods are straight. Confirm the CSC plate is current if sea freight is anywhere on the horizon. Then work out delivery, because a tilt-bed truck needs clear run and firm ground and a crane lift needs space overhead. Anyone buying an intermodal container for the first time underestimates the site access problem far more often than they underestimate the box.
