You need to include the definition of LTL: less than truck load, weight or bulk size. West of the Mississippi river we can pull 53 foot trailers, I imagine east of the river is still stuck with 48 footers.
Many of the shipments to the Walmart distribution centers consist of several pallets of product, not complete trailer loads. And even if the shipment might fill a rail car, rail freight is not responsive enough to satisfy most of Walmart’s needs. So most of the freight into Walmart is handled by truck.