Trade on the Long Mississippi happened both ways. The goods flowing towards the datum were mostly raw materials, such as timber, foodstuffs and minerals, and mainly came from the Corn Belt worlds. The staple export from these Earths were sacks of corn, in warmer areas sugar and oranges were also grown.
The Corn Belt also traded timber, which was the most popular spectacle with tourists in this area. Wood would be floated down one Mississippi or another, lifted out of the water by a twain, and roped together into huge platforms that hung beneath it. It would be taken to the Datum like this, with the troll labourers and their human supervisors riding in huts on top of platforms.
In worlds further than 500,000 steps from the Datum, only rare or precious items were worth bringing in, such as unique old-tree lichens, and other exotic flora and fauna.
In exchange for all these things, the Datum shipped out low-mass but high-tech goods, such as medical kits, electrical generators, and coils of fibre-optic cable (for establishing communication networks in new worlds). The Low Earths also traded items with worlds further out, especially horses.
There were waystations spread stepwise and geographically all along the Long Mississippi, where twains took on cargo. They were more frequent closer to Datum Earth. The stations had inns and the like for rest and recreation, and many a small jailhouse.
After the catastrophic winter of 2046, most long-range Long Earth exchanges, including most of the twains on the Long Mississippi, were stopped. This meant that there were less opportunities for people to become twain drivers.
In 2070, Joshua caught a commercial twain from the Low Earths to Valhalla on his way to a high meggers world for a sabbatical. The twain he caught was one of the few twains still sailing the Long Mississippi run.