Try to keep up? Who are you to advise nonsense to me when there are clearly no mention of track sharing in any of UITP, USDT or APTA's references. Please read the references if you haven't done so. In fact, you have clearly not kept up with the sources that were posted a long time ago -  and  just to name a few. Again this is gong nowhere without a single source verifying anything here. Like you say, a source trumps everything so you better stop inventing nonsense claims like "Seoul Subway Lines 1-9" that is non-existent in the very reference you quote.