I can't quite see the difficulty. Let's look at the train first: passengers are moving at the same speed as their surroundings but they are all held on the floor by gravity. If the train lurches, or brakes or accelerates then they will feel other forces, but will always feel gravity. So far so good? Or not?

In a falling box of any description (plane, lift, elevator, tea-chest) the contents are also moving at the same speed as their surroundings. The difference is that gravity is also moving the "floor", or whatever part of the container constitutes a floor. It's basically sky-diving but without the wind.