Plane plot foiled?
Unless Q is simply moving to 'trains and buses' a priori / without direct 'bus' or 'plane' evidence or chatter…
maybe this indicates desperation, in that trains and buses have different vulnerabilities and 'easiest' methods.
It would be easy to get a bomb onto a bus, basically trivial, but a little harder to pull off with a train, and there is a passenger list.
A train would be easy to attack like with the garbage truck straddled on the tracks a few months ago.
They don't even know what tools they will use. They are scrambling without resources or preparations.