40 foot well cars have several uses, involving 96 inch wide " Ocean " containers. Both BNSF & U.P. run garbage / dirty dirt trains from Puget Sound to landfills on both sides of The Columbia River in Eastern Oregon. A common configuration involves two - 20 foot open top former ocean shipping containers filled with dirty dirt in the well, with a 45'-48'- or 53' trash container on top. U.P.'s trash train includes dirty dirt gondolas as well.
The double - stack trains I've seen at Port of Tacoma usually have quite a few 40 foot or 45 foot well cars, as the steamship lines cannot handle containers longer than 45 foot on board. While many Intermodal well cars are designed for both 96 inch wide x 20',40', or 45' ocean cans and 102 inch wide x up to 53 foot long domestic containers, why have more dead weight than is needed for the load being carried ???