Apparently, they don't actually have electric trucks. Instead, they have "pods" for individual pallets:

"Innovation is also a significant feature of Cargo sous terrain because it will for the first time be economically viable to transport small volumes on individual pallets or containers on an ongoing basis. The continuous flow of small-component goods obviates the need for waiting times at transfer stations. In addition, the space requirement can be massively reduced because temporary goods storage is no longer necessary"