On the accelerating train, passengers perceive that they are acted upon by a force in the rearward direction pushing them back into their seats. We call an apparent force such as this one a fictitious force. A fictitious force appears to act on an object in the same way as a real force. Real forces are always interactions between two objects, however, and you cannot identify a second object for a fictitious force. The fictitious force is due to an object's inertia when the reference frame does not move inertially, and thus begins to accelerate relative to the free object. The fictitious force thus does not arise from any physical interaction between two objects, but rather from the acceleration of the non-inertial reference frame itself, which from the viewpoint of the frame now appears to be an acceleration of the object instead, requiring a "force" to make this happen.