An intermodal container is the steel box that quietly holds global supply chains together. Load it once at a factory overseas, and it can ride a ship across the Pacific, transfer to a train at a coastal rail ramp, and finish on a truck at a warehouse in Kent, Washington — all without the cargo inside being touched. That single idea, a container that moves across modes as one unit, is what makes intermodal freight cheaper and more predictable than breaking down and rebuilding pallets at every handoff.
This guide covers what shippers in the Kent and Seattle area need to know about intermodal containers: the sizes that matter, how the modes actually connect, and where drayage fits in at the Ports of Seattle and Tacoma.
What Makes a Container "Intermodal"
A container earns the intermodal label when it moves between at least two modes of transport — ship, rail, and truck — without being unpacked. The box itself is the unit of freight. The cargo stays sealed inside while cranes lift it from a vessel to a chassis, a truck drives it to a rail yard, and a stacker lifts it onto a well car.
The standards that make this work are set by the ISO, and they fix the corner fittings, the dimensions, and the maximum gross weight so that any crane, chassis, or rail car built to the spec can handle any container built to the spec. That interchangeability is the whole point. For a deeper look at the truck move that connects the modes, see our guide to intermodal trucking in Seattle.
Container Sizes Shippers Actually See
Most shippers work with a handful of sizes. Knowing which one your freight needs comes first, because the size drives the chassis, the rail clearance, and the rate.
- 20-foot standard. The workhorse for heavy, dense cargo — tile, metal, machinery. Two 20-foot boxes fit on one chassis and one rail well car slot.
- 40-foot standard. The most common box for general import freight. One per chassis.
- 40-foot high-cube. A foot taller than standard (9'6" vs 8'6"), for light, bulky cargo like furniture, apparel, and assembled goods. The extra height matters for cube-limited freight.
- 45-foot and 53-foot domestic. Used mainly in North American rail and truck moves rather than ocean imports. A 53-foot box is the domestic standard for intermodal rail service.
SeaSound Transport, based in Kent, handles container transport across 20', 40', 45', and 53' boxes, so the size you ship is the size that gets picked up.
How the Modes Connect at the Ports of Seattle and Tacoma
The Pacific Northwest intermodal chain runs through the Northwest Seaport Alliance — the combined marine terminals of the Port of Seattle and Port of Tacoma. A typical import move looks like this:
- Vessel discharge. A container ship arrives at a Tacoma or Seattle terminal. Cranes lift the boxes off to the dock or directly onto waiting chassis.
- Drayage. A truck carries the container from the terminal to a nearby distribution center, a transload facility, or a rail ramp. This first-mile move is where dwell time — how long a container sits waiting for a truck — makes or breaks the schedule. Drayage service that runs 24/7 pulls containers before congestion builds.
- Rail (long haul). For freight moving inland, the container loads onto a train at the rail ramp and rides to a destination ramp near Chicago, Memphis, or beyond.
- Final-mile truck. At the destination ramp, a local drayage truck takes the box the last few miles to the consignee.
The advantage is cost and consistency. Rail moves a container hundreds of miles on less fuel than a long-haul truck, and the container never opens between modes. The tradeoff is that the handoffs have to be timed — a container that misses its rail cut sits at the ramp until the next train.
Dwell Time: Where Intermodal Moves Win or Lose
The single biggest cost and reliability factor in an intermodal move is dwell time — how long a container waits at the terminal or ramp before a truck picks it up. Terminals charge storage after a free window (often four to seven days), and once a box is grounded and stacked, retrieving it takes longer and costs more.
A drayage carrier that runs 24/7 and holds a port pass keeps dwell low. SeaSound runs pickups around the clock out of Kent, holding the Northwest Seaport Alliance terminals close, so containers come out of the port on schedule instead of aging on the dock. For the cost side of the equation, our freight cost per mile guide breaks down what drives the line-haul number.
Choosing a Drayage Partner for Intermodal Containers
When you are moving intermodal containers through the Seattle and Tacoma gateways, ask:
- Can you handle the full size range — 20, 40, 45, and 53-foot boxes?
- Do you run 24/7, and do you have terminal access at the Northwest Seaport Alliance?
- What is your average turn time at the port, and how do you handle congestion or appointment-only windows?
- Do you offer transload or cross-dock at the warehouse, or only the drayage move?
- Are your drivers CDL-certified with current DOT physicals and a preventive maintenance program?
A family-owned drayage carrier based in Kent, with 10+ power units and direct dispatcher relationships, answers those questions with shorter turn times and fewer surprise fees than a broker juggling whoever is free. SeaSound Transport is USDOT-licensed and MC-authorized for interstate container moves, and you can reach dispatch at (206) 427-6615 or through the contact page.