Why shouldn't I ride the Metro? I know nothing of any dangers there. Should I not do my part to reduce greenhouse gas emissions? So what if I need to carry an assault rifle. Sign me up!
You shouldn't ride the metro because due to the staggering variety of mighty strange individuals who use it for their daily commute, you will likely see things that cannot be unseen, no matter the amount of bleech.
You shouldn't ride the metro cause some crazed phsyco who has a beef with John McClain is going to blow it up..
Or you might get shot by a mugger, or maybe they finally dug in the wrong place and released Godzilla.. or dragons.. or opened up the bottomless pit.
Or there could be giant mutated bugs with lungs who have human faces.
If you are a portable x-ray machine salemsman, you should definitely not ride the metro.. cause you know there are people down there just dying to steal it from you.
There are always zombies or giant rats in the metro.
And there could be somebody wanting a large payaout who takes over the metro and holds you hostage.
What about explosive sewer gases? A spark or flame at the wrong moment... and kaboom.. bye bye metro.