You're suggesting that because security checks are clearly impossible on mass transit systems such as London's, that would be sufficient reason to abandon them where they are possible and, arguably, desirable just because the risk affects fewer people?
Maybe we should also get rid of them for commuter flights with fewer than, say, a hundred passengers as that's not many people at all is it?
A regional sized jet is still one of the most powerful non nuclear weapons at humanity's disposal.
For instance, a Bombardier CRJ900 series airliner with 90 seats has an MTOW of something like 38300kg. Assuming it falls into "terrorist" control some time after the start of the flight it could still mass something in the region of 38t.
Its maximum cruise speed is something approaching 880km/h at altitude, which means it will likely maintain atleast that speed in a steep dive all the way to ground level.
880km/h is approximately 244 metres per second, meaning that the ~38t airliner impacts with approximately 1.14GJ of kinetic energy, and delivers a payload of something like 8000kg (assuming some fuel was already burned to take off) of Jet A to the target area.
8t of Jet A has a calorific value of roughly 320GJ, and can be expected to combust relatively rapidly after impact, if not in seconds then in single-digit minutes.
The impact alone is 300kg of TNT (and in the form of a kinetic penetrator rather than an actual explosion), and when this is mixed with the fuel yield.... you can deliver massive damage to the target area.
Even a detonation on a plane soon after takeoff could scatter vast masses of flaming debris all over some suprisingly heavily populated areas.
It is difficult to concieve of a similar situation that will lead to such widespread destruction from "terrorists" getting either weapons or explosives onto a train.
