The wall between banking and commercial enterprise in the US is crumbling in the high-stakes world of commodities.

A number of big international banks have in recent years won Federal Reserve approval to buy, sell and store much more than bars of gold in vaults. Banks have chartered tankers to park oil at sea, hoping to profit from future price gains. They have entered “tolling agreements” with power plants, selling them coal or gas and buying their electricity. Some own power plants outright.