bottom side rail

Two rails, one along each side, full length

Two. That is the count on every container, one bottom side rail running the entire length of each side wall, from the bottom corner casting at one end to the bottom corner casting at the other. There is no separate piece for a 20-ft versus a 40-ft box, just a longer single rail on the bigger container.

What changes is not the count, it is the exposure. Because the bottom side rails sit at the very base of the container, closer to the ground than any other steel member on the box, they are the first place moisture, mud, and standing water reach. A container that spends years parked on wet, poorly drained ground shows it here before it shows up anywhere else.

What the bottom side rails actually hold together

The bottom side rails do double duty. They tie the corner posts together along the base, giving the container its lengthwise rigidity, and they form the outer frame that every crossmember bolts or welds into underneath the floor. Between the two rails and the crossmembers running between them, that is the entire floor structure, before the plywood decking ever gets laid on top.

Why this is a place worth checking on a used container

Because a bottom side rail sits at ground level, it is also one of the easier structural members to inspect without climbing under the box, unlike the crossmembers hidden above it. Walk the base of a used container and look along the rail for heavy rust, pitting, or holes rather than just surface discoloration. Light surface rust on a cargo-worthy unit is normal and expected; rust that has eaten through the steel is a different conversation.