"What scares me is that it seems unlikely that all three bombs would be "duds."
I think it is most likely that the detonators went off but didn't spark the explosive. Quite likely if it was a homemade bomb like the ones last week seem to have been.
One eye witness who was in one of the tubes said a man came on the tube, placed a package (napsack?) on it and ran off. The other passengers did attempt to go after him but were unsuccessful.