I hadn't thought about it then, but isn't the reason the Shadow can get Trollocs through the Ways because they have the Waygate in Malkier? With Lan's Borderlander army blocking the Gap, how would they have made it there?
Elder Haman mentioned two steddings with Waygates that were overrun by the Blight in LoC. And there are other places where Trolloc armies mass. Tarwin's Gap is just one of the best, and Lan's sympathies lean that way because of the Malkier thing. That's also why Nynaeve made him promise to go to Fal Moran before the Blight, because he could have found a place to fight further west. In short, there's a lot of room for the Trollocs to mass and they don't need to go through Tarwin's Gap to do so.
“Tolerance is the virtue of the man without convictions.” GK Chesteron
Inde muagdhe Aes Sedai misain ye!
Deus Vult!
*MySmiley*