If a train runs every 10 minutes, what does it mean to say that there's a 10-minute delay all day? It presumably doesn't mean that all trains are delayed by 10 minutes, since then you'd simply be taking a different train at the same time and not notice or care. Perhaps it means that the inter-train interval has increased to 20 minutes, but is that really a 10-minute delay? Or maybe the average wait has increased by 10 minutes, so the inter-train interval is now 30 minutes. I don't think that works either.
We'll ignore for now the question of how there can be a delay on only part of a line.
