Evolution of hot and dense nuclear matter produced in central Au+Au collisions at energies of NICA, FAIR, and SPS is studied within two transport models. Two interesting features in the matter behaviour are observed almost from the very beginning of the collisions, at t ≥ 2 fm/c, for all studied reactions. (i) Expansion of the system proceeds with constant entropy-per-baryon ratio. (ii) Effective equation of state has a linear form, P = aε, a sime const. Both observations support the formal application of hydrodynamics at the early stages of heavy- ion collisions, when the system is very far from the equilibrium.