Invasion by homeless people on trains
An Apocalyptic event could conceivably be brought on by the homeless on trains. Their bites are excruciating and ultimately fatal, so the usual containment measures for other existential threats to mankind like zombies, overlord aliens or Finns will not suffice. In case of attack by a homeless person on a train, the first thing to do is avoid being bitten. Their venomous fangs need only scratch the skin lightly to bring on howling pain.
The homeless have long been more than an annoyance, but less than a nuisance in first world countries for hundreds of years. Most of us know this much. Clearly the first line of defense against these miscreants would be the train conductors. Department of Defense officials state that all train conductors will now receive intensive training in the containment and application of lethal force of the homeless. This will be the same training given to Special Forces, Marines and the Coast Guard. Navy chaplains are also to receive this training, to be supplemented by vitamins and alacrity.
Other dangers from the homeless
The invading homeless on or from trains may also attack the genitalia of the unwary. Most frequently assaults are carried out using umbrellas, half-tracked vehicles, horseshoes and bacon.