I know it's no fun if you're in such an accident, but in my own strange way I've been keeping track of accidents that take place within a single transit agency, i.e. a light rail vehicle hitting a bus operated by the same agency.
Of course there have been many incidents of one train hitting another, but this is limited to what happens at grade crossings between a train and a bus.
In no particular order:
Philadelphia, August 2007: SEPTA trolley vs. bus
Los Angeles, September 2008: LACMTA/Metro LRT vs bus
Los Angelee, July 2010 LACMTA/Metro LRT vs bus
Los Angeles, August 2012 LACMTA/Metro LRT vs bus
Weehawken, NJ, August 2006: NJ TRANSIT HBLRT vs bus
Phoenix, AZ, June 2009: Valley Metro LRT vs bus
Houston, TX, February 2010: Houston's Harris County MTA LRT vs. bus
The accident in March 2009 between a MARC train and a Metrobus in suburban Washington, DC doesn't count as it was two different agencies.
Pennsauken, NJ, December 2009: NJ TRANSIT AC Line vs bus
Oradell, NJ, May 2012: NJ TRANSIT Pascack Valley Line vs bus
Minneapolis, August 2008, Metro Transit LRT vs bus