He could have arrived at King's Cross on the eastbound Circle Line
train, leaving the FIRST bomb on that train.
He could have placed the SECOND bomb on the westbound Circle Line train
without actually riding it, just getting on, leaving the bomb and
getting off.
Same with the THIRD bomb. He could have placed it on the Picadilly Line
train without actually riding the train. They say the bomb was on the
first car, right where the escalator from the lower level would have
brought the culprit. He placed the bomb on the train and then took the
escalator to the surface.

Your scenario may be plausible from the "time between different trains point
of view", but don't you thinj, that the the following might be a little
suspicious?
That would mean for bomb 1 and bomb 2 that he entered the train, preferably
as first person of the entering crowd, placing the bomb somewhere and
leaving the train at the very same station, which is KX, if I got your
theory right.
How long do trains stand in stations in general and in KX in special on
average. I guess somewhere ranging between 20s and one minute. Including the
expectedly large crowds on the platforms and IN the trains, that would put
some additional stress. Many people in the train, probably no seat to claim
- for placing the bomb beneath - just hopping on, trying to appear calm and
place what kind of bomb on whichever place and getting off at the same
station - probably hassling through the people that entered the train after
him - without causing attention ... well I don't know!

How big could a bomb have been? Was it more like the size of cell phone
(highly explosive stuff) or somewehere at the briefcase/rucksack size
(probably a bigger amount of less brisant explosives).

I can hardly imagine someone forgetting a bag in this short a time in the
small carriages full of people with nobody seeing it and getting
curious/suspicious or simply helpfull and trying to get the bag to the owner
back.
But it may be possible if London's tube-riders are as sleepy or less caring
as in Vienna.

[...]
Verify that it is IMPOSSIBLE for it to be the work of one man before
looking for more than one person!

Do the trains of all these involved lines so exactly on time, that the one
person could have relied upon it just by gathering the timetable information
through train-use and writing down the time. Or would it be required to have
an "inside view"?

thanks, very curious from Vienna
Tadej
--
.... aber auch bei Frauen hat das Großhirn tatsächlich eine Funktion ...
Selbst im wildestens Winterschlußverkaufrausch verstehen sie noch das Wort
"Kreditkartenlimit".
David Kastrup in d.t.r