Era 5 Engines

Researching just the solid lines on engines will get you to the next engine quicker. Unless otherwise noted or required, I don't recommend researching all engine upgrades.

First: The UNICORN Path is the quickest way to the CENTAUR researching only the required upgrades. It uses 4 fewer research points than Path MEDUSA. Some prefer Path MEDUSA because of its speed. Your choice.
UNICORN: If you don't have Era 4 Coupling and you need an Era 5 engine quickly the UNICORN is the first engine in its Path. The UNICORN has 7 cars with an upgrade and a Speed of 160 km with 2 upgrades. But the UNICORN's base Acceleration of 4/20 is low.
MEDUSA: If you don't have Era 4 Coupling and you need an Era 5 engine quickly the MEDUSA is the first engine in its Path. The base Speed of 210 km and base Acceleration 12/20 makes it a fast engine. Although it only has 5 cars (6 with an upgrade), the Speed and Acceleration still makes it an engine used often. Especially for players who didn't get Era 4 Coupling.
BASILISK, SATYR and LEVIATHAN: Used by players who researched Path UNICORN to fill in slots when they don't have enough open slots to get another CENTAUR or Era 6 engines. Each has it's advantages and disadvantages. But they are not often used except as fill ins. So I won't go into the details of each.
CENTAUR: The best engine of Era 5. While the CENTAUR's Speed of 135 km (with 2 upgrades) is a little slower than the POSEIDON, its top acceleration of 20/20 (also with 2 upgrades) is much better. The CENTAUR can haul 33 cars with 2 upgrades. Only the OLYMPUS (Era 6 engine) can haul more. Getting to the CENTAUR should be your highest goal in the research area for Era 5.