The aluminum vs steel decision has much less to do with fuel consumption than it savings associated wtih lower tare weights. A lighter aluminum car allows the shipper to load that much more without exceeding the gross weight limits. And the more product you put on a train, the fewer trains you have to run, which makes the railroads happy. Stainless hopper cars have also gained some favor for the same reasons, as stainless cars can be made lighter than carbon steel cars. Aluminum is more expensive on a per-lb basis, but it ain't 30x steel... I think it's more like 2-3x steel.
Aluminum "sheet and post" domestic containers are fairly common (many EMPU containers are alum). However, I'm not aware of any shipper/carriers buying aluminum now. The Chinese-built 53' corrugated steel box is now the container of choice. Like a lot of stuff from China, they're cheap, rugged, and easier to repair than aluminum. There is a slight tare weight penalty (about 1k lbs), but as most intermodal shipments cube out before weighing out, this isn't a big issue.
Why aren't there aluminum boxcars? That I'm not 100% sure, but I'd reckon it's a number of issues: Boxcar purchases are hard enough to justify at steel prices; Cube capacity is typically the issue with boxcars, now that 100- and 110-ton cars are available; boxcar utilization is far from stellar, so you'd be paying a premium on equipment that doesn't turn a lot of business.