A figure 8 with 18" radius curves will not fit in a 6ft length. You need at least 6 -1/2 ft for the 2 turns plus the distance from the centerline to the outside of the track plus at least 2" cushion at each end for the trains to come off the track and not fall immediately to the floor.
The practical continuous running arrangements in 4x6 with 18" radius are an oval with a couple of spurs and a passing track, or up-and-over folded figure 8 (also called a twice-around). If you want longer trains, a double track oval is feasible.
I googled your "Warbonnet Express" and came up with an Athearn train set featuring an F-7A, three cars, and Bachmann EZ-track. As others have pointed out, the EZ-track while great for running on the floor or carpet, does not have the flexibility most modelers want in designing and building their layouts.
If I were your architect, and you were commissioning me to design your house, I would insist on a lot more information about what you are trying to achieve, the intended purposes of the various rooms, and your preferences in architectural styles. You would have grounds to sue me if I didn't ask those questions.
Similarly, a model railroad is usually a very rewarding, but not insignificant investment in time and money. I have heard figures of $50 to $100 per square foot, and a year to bring a 4x6 to a reasonable level of completion with scenery and structures. And what you want to have is a very personal thing in your mind's eye. An example is your desire for a bridge, and a track that crosses through the center of the layout.
I was very surprised that you mention a son in one of the later posts in the thread. How old is he? Are there other children? Is it his layout and trainset, or yours? All this makes a huge difference in what my recommendations would be.
Priorities would be the bridge and track crossing through the center. If I could deliver both in a design that took 4x8 instead of 4x6, would you have the extra space? Are you willing to learn to use sectional and flex track, and lay your own roadbed to get more of what you want in a given space? Is the bridge more important than switching opportunities? Are these relative priorities likely to change (that is will you get tired of watching a train run continuously on a twice-around up-and over)?
The smaller the layout, the more focused your priorities must be because you have room for so few of them. I urge you to click on a few of the links you have been provided, and give your reactions to some of the suggestions and questions.
yours in planning