They won't. Anything on a bullet train line needs to go at a speed of a bullet train. If it goes slower, it slows down the bullet trains on the line.
Cargo does not require bullet train speeds, only passengers do. The added expense does not translate to a better service. Cargo is not time sensitive like people are, so usually freight trains go under 100 km/h. This requires a whole lot less infrastructure and a whole cheaper locomotives and wagon compositions, as locomotives and wagons that go faster are more expensive and require more maintenance.
I don't know why OP is getting downvoted. This comment is correct. The economies of freight on rails are predicated on scale. The bigger the train, the better. It only requires a few locomotives that have the biggest maintenance costs. Cars are unpowered and require less maintenance. A train can transport thousands of tons. An average mid-size train in Europe would carry about 2000 tonnes of freight.
To add an electric motor per ton of cargo transported (as detailed in the article) adds an excessive amount of overhead costs to the cargo transported, in upfront costs, maintenance and environmental impact, making it less competitive than a regular freight line. With the space of the same infrastructure and probably the same investment, you could instead run a freight line, especially considering the distance between Osaka and Tokyo. Over time, the freight line wins out over the individual one ton self-propelled cargo wagons. Remember, every propulsion system breaks down and requires new parts and fixing. The more of them there are, the more complex your transportation system becomes, thus more expensive. An equivalent to a mid sized freight train in pods would require about 2000 electric motors, as opposed to one or two dedicated electrical locomotives.
They should instead improve the trains to be able to run mostly automated. This requires track sensors and advanced signaling.