Trailers will have posts for the shipping containers to seat nicely on. I don't believe there are fasteners.
How exactly are freight containers fastened on the tractor trailers behind large trucks?
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Containers are secured to the truck's tray, or frame on a skeleton trailer, by standardised
twistlocks.
You'll see that each corner of a container has a socket, with several slotted holes, one facing downwards. This image shows a 40' container.
The truck has a set of fat, stubby
oval pins set into the truck. These pins are usually retractable into (or can be pushed above) the bed of the tray or frame by about four inches. The pins have two sections on a vertical shaft.
Some trucks have several sets to allow correct positioning of different loads: i.e., 1x20' in the centre of a long tray, 2x20' containers or 1x 40' container, each of which use different pin configurations.
Before the container is dropped onto the tray, four twistlocks are pushed up by the driver, to leave a pin protruding vertically from below for each corner of each container.
The container is placed into the pins from above. (These images show non-retractable twistlocks, but you get the idea, no?)
The handle of the pin (below the truck body or frame) is turned and that action rotates a shaft passing up through the frame. This turns the top inch or so of the oval pin by 90deg.
These rotated tops stops the container from lifting more than a few millimetres above the frame or tray.
The twistlocks look small and are certainly inconspicuous when in use as they are concealed by the container, and when retracted are mostly invisible. Everyone I've shown them to has been surprised at what little (apparently) holds a container to a truck but if you look at the shaft, it's a good inch-and-a-half of steel, multiplied by four.