hmm ...not sure if this is the issue here but, if you type in a starting station for one region with the same ending station as the previous region, you'll get an error that the regions overlap. If you put in a gap between the end of one and the start of another you'll get just that, a gap in the corridor. So, what I usually do is 'pick' the locations of where one ends and the next starts. They'll appear as the same station in the corridor but somehow they're interpreted different and you don't get the error, nor the gap.