Sadly, these aren’t even the first fatalities in recent memory on the DC Metro transit system - a couple of years back, there was a string of fatal bus-on-pedestrian accidents. This is the first fatal Metrorail accident I can remember (though I’ve only been in DC for a few years, so I’m not the best source)...
Metro has recently had a huge budget deficit (despite raising the price of everything), and has started skimping out on maintenance. Today was a relatively hot day, and my guess is that something on the track popped, causing the wreck. If so, then there will be big time hell to pay.
When I worked at Catholic U, there was a young law grad who was killed in a Metro crash. I think 3 people died in that one; it would have been in the 1980’s.