I connected the different modules by drilling holes in the wood frame at the connecting ends of each baseboard and then inserted bolts and nuts to connect them together. Baseboards are a maximum of 6 feet long.

If you are planning to exhibit your layout or want a layout baseboard that is stronger than wood, you might want to consider a steel frame, and still use bolts to fasten the baseboards to each other. I know a few model mailroaders who have done this for their exhibition layouts, and it works very well.