Consider the usual example of relativity - train and the observer on the platform. Now, the observer is moving according to the train and the train is moving according to the observer. But we know that it is the train which is spending the energy to move forward. We can neglect the friction and just consider the ideal linear motion of the train on the tracks. So, if we consider two systems: train and the observer, then it is only the train which is doing the work. So going by this, why cannot we say that the statement "observer is moving wrt train" is false and it is only the train which is moving wrt the observer since the observer is not doing any work?