When trying to place a Minecart on rails, it cannot be done if an item dropped as an entity is present and sitting on the rails. As soon as the item entity is picked up by player or despawns the Minecart can be placed on the rails.

Not sure if intended due to hopper Minecarts or chest Minecarts but I think it might be a bug. I have only tested on a Realm server with standard Minecart (not hopper or chest).