Against the back wall. The only reason we move forward is the resistance from the seat we are on.

Is the train accelerating? Or is it staying at a costant velocity?

It will move to the back as the air compresses slightly in the rear but as soon as the air re-equalizes it’ll stop going backwards. The air provides an (almost) closed system inside of the train car that makes its inertial frame (almost) the same. Perfect case if the air is incompressible it doesn’t move at all, another perfect case where there’s no air it hits the back. but this isnt a perfect case so it’s somewhere in between.

Many have cited inertia, that would not apply here as the problem states the train is at rest when the drone flight initiates. The drone would strike the back wall. If the train was in constant motion when the drone flight initiated, it would remain in motion with the train.

Smack against the wall in the back

Americans don't know about relative motion I see

People are super simplifying this: When the train moves the inertia of the copter will first appear make it go backwards in the cabin, but at the same time the air within the cabin also moves, gradually applying pressure onto the copter and accelerating it. But whether the copter will gain enough acceleration before it smacks the wall, or in other words aceheive the same speed as the train before it runs out of room needs experimentation.

The drone would shift slightly to the forward side of the cabin of the train, few seconds after, it will hover and won’t move at all unless you control it to move. This is due to the first few seconds when the train accelerates, the air pressure of the cabin will shift to the rear, causing a forward force to push the drone forward a bit since the drone hovers due to air molecules in the cabin. See video in the next comment.

Hit the back of the train Since the helicopter is hovering and doesn’t move in the same direction as the train as the train starts to move

Smack the wall