Monsters are often called mobs (possibly from “mobile object”). A train is a whole group of mobs, which together are far more dangerous to PCs than a single mob; when someone nearby yells “Train!,” you’re best advised to run like hell. In verb form, to train is to lead a train, that is, flee ahead of it—this is usually due to misfortune, but sometimes a character will seek to lead a train into a group of waiting comrades, as a means of ambushing the mobs. Thus “That guy is training the skeletons” doesn’t mean he’s seeking to improve their skills, but instead leading them a merry chase across the landscape.
Thankfully, the bulk of the words aren’t new to me, although I’m far from fluent. I’m still down with the kids, daddio. Incidentally, the article’s taken from Verbatim: The Language Quarterly which has a load of back issues online as PDF. I’d never heard of the journal before but it’s chock full of goodness for those with an interest in word usage and origins.