The attacker chose his target intelligently. If he hadn't have been stopped, this could have been horrific.
If he had attacked a cinema or a shopping mall with multiple exits, people would disperse and flee very easily and quickly as soon as he started shooting. Armed police would be on the scene in minutes.
On a train, hundreds of people would effectively be trapped in there with him until it could be brought to a halt and the doors opened. He would have walked the length of it, killing at will. This would have been worse than Anders Breivik's attack. The two that stopped him averted a nightmare.
Of course now, there'll be talk about airport security at railway stations. The UK has over 2500, including many small ones used by less than 100 passengers a month. So that's going to be a problem.