Get up on the building next to the parked train - there is a ladder in the inner corner between building and train. Jump down on the roof of the wagons, and enter through the roof hatch.
Also notice, in spite of this supposed to be Germany in WWII, the wagons have American couplings... :)

In the storage, it is necessary to jump over to a platform. It's a long jump, but doable. Please be aware of the splat factor, though.