The Cargo Cart is uncraftable in Railcraft 12.0.0 for Minecraft 1.12.2, since the recipe, Trapped Chest + Minecart, seems to be conflicting with the recipe for Chest Cart, which accepts both normal Chest and Trapped Chest.

Still have this issue

Sadly, the Railcraft project has not seen any (public, at any rate!) development work since mid-March 2019. At this point, your best bet for solving this is to modify the recipe yourself via CraftTweaker or the like.