One of the new Bombardier streetcars has derailed on King Street while it was pushing another older model streetcar which had also derailed. A spokesman said the new streetcar derailed because of debris in the tracks left by the morning rainfall. Both streetcars were empty at the time. The lesson for anyone who thinks about this is not so much that streetcars can derail (we wish they didn’t). No, the unbelievable profligacy of streetcars is this. They require an entire stand-by system of buses to take over when one — just one — streetcar blocks the tracks. The shuttle buses were in service Tuesday afternoon. Bus systems are flexible. A bus can sit idle by the side of the road while the others continue service. They don’t require a back-up system of streetcars to replace all the buses.