There are no concealed carry permits in DC. The DC Metro has recently started random searches of passengers. The DC Metro authority apparently wants to make sure that the small minority of white professionals riding their trains are no danger to the gangs of feral black youths roaming the platforms and beating people for entertainment.
The feral youths mostly don't carry briefcases and conceal whatever they want withing their oversized hip-hop clothes, and are not subject to searches because "that be racist".
You’re correct. The only people carrying guns on the Metro are those with criminal intent - or who need to protect themselves from unhappy brothers.
Which is exactly why this event happened. With the threat of swift, deadly justice, criminals will go elsewhere to seek their thrills.