"Mötas" and "träffas" can both be used to describe that two people meet:
Vi möttes på torget / Vi träffades på torget
"Tågen möttes" is a correct sentence to describe that the two trains were running in opposite directions, on different tracks :), and passed each other. Here, träffades does not work since we are not talking about living beings.
"Jag träffades av tåget" actually means that you were hit by the train. You can also
träffas av blixten (lightning)
träffas av en kula (bullet)
Yes, there is a full set of verbs ending in -s despite not being passive. These are called "deponent verbs". However, träffas is actually just the passive form and those pretty much always end in -s, so it's not a regular present tense.
Then there are also a couple of irregular ones, some of which you've probably seen - like jag vet, du kan, etc.