In the wake of other train crashes, it has become common to rule out human error first. In fact, it's far easier to address that first because mechanical failures take a lot longer to investigate. After the DC Metro train crash two weeks ago, one of the first things that NTSB released was the fact that the train operator's cell phone was off and in her backpack and that the emergency brake had been pushed and was engaged. I'm not at all suggesting that human error is the fault here, but it's not unreasonable to consider that as a possible cause.
That said, I've seen a lot of discussion about how a mechanical failure is impossible on the Monorail. That was what Metro said as well; that it was a fail-safe mode of transportation. Relay-switches are meant to keep trains a certain distance from each other. As it turns out, maintenance was performed on one of the relay switches 5 days before the accident and it failed in such a way that it was virtually undetectable until the crash. The investigation is still on-going, but my point is that none of these systems are really fail-safe. But safe enough that I have no issue using the Metro system every day. And, hopefully, if the Monorail is up and running when we visit in two weeks, using it as well.
In the meantime, my thoughts are with the driver and his family. What a sad sad day for everyone involved.