This is the 2X6 base frame. It is designed to be very rigid to support the upper frame and track, and possibly support lifting the track to the ceiling (wench system). You can also see how it is designed to be separated into 3 parts for removing the track from the room when necessary. The legs are round (blue).
This is the track framing. This will provide a platform for the plywood top and racing / scenic surface. It's also designed to be split into 3 parts with the base.