Thanks for the feedback Laurent...
Brian, here are some additional external links concerning this incident:
Roll of Honour – Rennes troop train bombing, 17th June 1940
Rennes, Brittany, France, June 1940. After Dunkirk, escaping to the west. Chapter 1
Rennes; 17 juin 1940, tragédie dans la gare à la plaine de Baud (WARNING! includes graphic images)
I also agree that this appears to have been a target of opportunity, so perhaps it was one of the two units who recorded losses (both from "The Battle of France..." tome) on this day?
1./KG26 - Heinkel He111
3./KG30 - Junkers Ju88A - possibly aircraft that attacked the Lancastria
Hopefully others can add more info...
Cheers
Peter D Evans
LEMB Administrator