The tank-car retrofit market never fully materialized, Greenbrier and Watco leaders say. Photo – The Greenbrier Cos. Inc. The Greenbrier Cos. Inc. and Watco Cos. LLC have agreed to discontinue their GBW Railcar Services (GBW) rail-car repair joint venture, the companies jointly announced yesterday.Under a dissolution agreement, the repair shops and employees at each GBW…