Suppose a reference frame S1 is accelerated in the x direction with a constant acceleration g. A second frame S2 is at rest in a homogeneous gravitational field which imparts to all objects an acceleration −g in the x direction. Observationally there is no distinction in any respect between S1 and S2. All bodies are, to wit, equally accelerated in the gravitational field. By GENERALISING this mechanical equality, Einstein postulated the complete physical equivalence of a gravitational field and the corresponding acceleration of the reference frame. This is the so-called ‘equivalence principle’.
The first realisation of equality is from the Einstein Elevator thought experiment. Could someone please explain what grounds Einstein had to generalise his local observation to conclude with the equivalence principle.