According to their rules, everything has to be perfect before the line opens. If it were as you suggest, the line would have opened years ago. Of course, once it's open, if something causes part of the line to close, the rest of the line can be kept going, just as it does all the time on the subway system. It has to do with liability, and what's written into the contracts. I believe many of us on here, think these rules are dumb, and need to change for future projects, because it's looking like the current rules make it impossible for a new line to ever open.