Yes, it is extremely bad when 60-100 ton railcars are being thrown around like toys, at nearly the speed (about 30m/s slower than most .45 ACP) some handguns fire bullets.
*as you can with a _plane_
Setting aside hijacking, which would not be the mechanism for a train attack anyway, the more likely attack on an airplane is suicidal self destruction at this point I would think, with all the new mechanisms in place.
If TSA is this silly with planes, given the relative lack of attractive plans open to attackers now, what makes you think they won't get even sillier with trains than they already have.
As for hitting a specific building, that depends: does your high speed rail line pass near any attractive targets? If the train is 800,000kg (not unreasonable I think?) and is travelling at the rated 280m/s or thereabouts, that's about 31GJ (please check my math) or 7.5 ton TNT equivalent of sheer momentum that can be convinced to continue travelling in an undesirable direction.
Seems like it may be a bit of a risk, at least enough that somebody could be convinced to thoroughly screen passengers.
That's a problem that will be replicated exactly for boarding a train destined to travel at 278m/s.
The consequences for a major security failure on a high speed train are, if anything, more dire than those for even a very large passenger aircraft. An 800,000kg barely-subsonic chain of rigid torpedoes barreling through dense population centres is serious business.