They take turns assisting lockages, usually six tows up then six tows down. A standard hopper barge is 26' x 175' and has a double rake. A jumbo hopper is the most common on the river and is 35' x 195'. Several companies have started building 200' jumbos. Stumbos and jumbos are built either single rake or box style, though there are some double raked stumbos out there.
Picture shows a single rake jumbo hopper with fiberglas lids to cover the load.
Here's a couple of barges used to transport concrete to sites requiring a lot of cement. Here's an interesting barge. This is an A frame used to salvage sunken boats and barges. Here's an innovation. A shortened hopper barge converted into an A frame.