Since the venting system is designed to protect the containment, it must release gas from the dry well and perhaps also the top of the torus. From there it would go to the filter "train", then outside the building along the fat pipe to the stack.
According to a NYT article the hardened venting system doesn't use filters, the regular venting system does.
They were also using "train" in the sense of several connected filtering systems (like wagons on a train) when talking about the Areva water treatment system. I think the train room holds several filters trough which gas would sequentially pass before being released into the stack.