The below is what i found the best order for engine change in my few days of game play in multiple servers. Share your thoughts.
Goods -> Swallow - Raven - Donkey - Black Bear - Dionysios - Morphues - Thor - Medusa - Centaur - Olympus
(Traction giants on first 2 ERAs, then move to sprinters and finally move to multi engines for end game where wait times are longer and may be hauling from warehouses)
Passenger -> Giraffe - Ajax - Hermes - Aten - Pluto
(No passenger trains in ERA 1 to be bought and no unnecessary tracks to be laid, rather focus on engine house to level 7, Laboratory, Bank, restaurant and shopping center alone in ERA 1)