minecraft-assets/data/minecraft/recipes/minecart.json

{
  "type": "minecraft:crafting_shaped",
  "category": "misc",
  "key": {
    "#": {
      "item": "minecraft:iron_ingot"
    }
  },
  "pattern": [
    "# #",
    "###"
  ],
  "result": {
    "item": "minecraft:minecart"
  }
}