Wheel, cart and road, then rail and the internal combustion engine.
Move heavy things further than you can carry them.
Needs first: Woodworking and the first machines (Tools), Blacksmithing (Tools), Livestock (Food)
Move mass cheaply overland.
Needs first: The steam engine (Energy), Blast furnace and cheap iron (Materials), The factory and interchangeable parts (Tools)
Put a compact engine in a vehicle.
Needs first: Petroleum and refining (Energy), Machine tools and precision (Tools), The generator (Energy), The factory and interchangeable parts (Tools)
Every guide, build, reference sheet and book reached from the steps above.
The first machine tool — a sapling, a cord and a treadle, and the direct ancestor of everything in tier 3.
A few kilowatts, day and night, without eating — the first energy surplus large enough to support full-time specialists.