there's no official training that i'm aware off (except when the change from ICL to WoWPoS came in)
the usual training method in our store is to spend a few weeks showing somebody the ropes, with respect to refunds etc. the common stuff, then move on to having them do 1 or 2 "buddy shifts" with another supervisor, then throw them into the deep end with an easy shift on their own.
edit: however, rarely is the more complex stuff taught, omly if and when needed e.g. Manual EFT, EFT Settlement, Dissection dumps