Most of us are familiar with the containers used to transport a variety of cargo—from consumer goods to agricultural products. Last month, Terminal 6 received a special delivery: more than 400 empty containers that were the cargo—new boxes put into circulation as part of the global supply chain.

These containers are for domestic shipping, moved by rail, and are 53 feet in length, unlike the standard TEU or 20-foot-equivalent unit container used for international ocean shipping.

Here are seven more facts you may not know about container shipping:

1. Moving products by rail is environmentally friendly. Rail can move one ton of freight an average of 468 miles on a single gallon of fuel.*
2. Approximately 97 percent of shipping containers are made in China.
3. Every single shipping container has its own unique identification number—just like a license plate. This allows containers to be tracked wherever they travel.
4. If a shipping container receives regular maintenance, it can stay in circulation for about 20 years.**
5. Containers are made of corrugated steel for strength and can be stacked up to six high.
6. The intermodal containers delivered in this Portland shipment will stay in the U.S. for domestic cargo, making the rounds across the country via Union Pacific Railroad.
7. Fun fact: Singapore was so thankful for the creation of the shipping container, it was featured on the country’s $1,000 bill until 1999.***

* Source: Association of American Railroads

** Source: Container Auction

*** Source: Quartz