According to Vicky Hutchinson from the Department of Transport the tube trains in question on the 7th July - the 204, 216 and 311(331) left Kings Cross at 0835, 0842 and 0848 respectively.
The media, presumably based on police statements, have published a variety of train times relating to the journey the four men reportedly took from Luton to Kings Cross Thameslink. The 0740 and 0748 are the most referenced. Information provided by Chris Hudson from Thameslink Rail has shown that the 0740 had been cancelled and the 0748 did not leave Luton until 0756 arriving at KC Thameslink at 0842. The four men are seen in the cctv shot entering Luton at 0721:54. The only train the men could have travelled on which would have got them to KC Thameslink with enough time to board the tubes would be the 0725.
See government references to the train times in the reports analysis
Surely this information is easily attainable - if the men did indeed take the train that morning from Luton to KC Thameslink they would have been filmed on dozens of cameras.(images)
Media Reports of train times
Telegraph: Police appeal for bus bomber information (0740)
Times: CCTV pictures show London bus bomber (0740)
Independent: 77 What really happened ? (0748)
BBC: Horizon - wmv (0748)
Research into actual times
Thameslink Rail Limited: Chris Hudson (trains)
Department of Transport: Vicky Hutchinson (tubes)