Whereas a dungeon chest does not generate its loot before it's opened (go into a new world, find dungeon chest, blockdata it, see it has LootTable rather than items), this is not true of abandoned mineshaft minecart chests. They populate on world-generation. You can witness this either by

Finding MinecartChest in a new world and entitydata-ing it, or

Finding MinecartChest in a new world, save & exit, add a data/loot_tables/minecraft/chests/abandoned_mineshaft.json to override it, reload the world (or F3+T), and open the chest. It will still have the 'default' loot rather than whatever you changed it to. But if you TP 10000 blocks away, new minecart chests will have the new data (populated as those new chunks generate under the influence of the new overridden loot table).

Expected: MinecartChest behaves like Dungeon chest, not populating with loot until there is an observer-interaction.