Let's assume a helium balloon is suspended into the floor of a moving train. We already know how it will react during acceleration, deceleration, and constant speed as illustrated in figure no 1, 2, and 3. However, what will happen to the balloon if we detach the balloon from train floor and attach something that is exactly the same weight as it's floating force thus making it "floating" in the train cabin. Assuming that we do that after train has stopped accelerating (moving at constant speed) as illustrated in figure no.4. Will it stay where it was detached inside the cabin or will it hit the back of the cabin as the train is moving forward at a constant speed?
First of all, weight which attached shouldn't be equal to floating force since rope and balloon have mass too. But let's say balloon is floating for us. Assuming that you have travelled with train at least once. You know that air molecules are not just back side of the train but everywhere. You can apply the same logic for floating balloon too.
Ideally, there will be no external force acting on the balloon in the horizontal direction so it will move along with the train in accordance with Newton's First Law.
However in reality, there will be collisions with gas particles present in the coach which will give it some random velocities.