On a whim, I checked the accident record in wikipedia (http://en.wikipedia.org/wiki/Seattle_monorail#Accidents) for the Seattle Center Monorail. Fairly harmless accidents-- and one incident of a wheel falling off and landing on a windshield (of a European tourist, IIRC) below seems to be missing. Fortunately nothing fatal, listed or not. The March 31st, 2004 accident omits that it was Monday, Memorial Day, and a warm one, at that-- Memorial Weekend is the weekend of "Folk Life," a free folk arts & music festival, one of the first major events of the season at Seattle Center and one that really brings out the crowds.

However, scale these accidents up per passenger*miles (considering that one or two monorail trains moves A to B, and B to A along a 2-mile track, every 10 to 15 minutes during operation) compared to the DC metro, and you've got one hazardous system. :^)