Age of Steam Expansion: Human Body / Synapses is a two‑map expansion that turns the classic freight‑running board into a living, breathing organism. The first map places your locomotives inside a stylized human body, where oxygen and carbon‑dioxide cycles dictate the flow of goods. As the game progresses, organs change color—blue for oxygen, red for carbon—forcing you to adapt routes and cargo choices on the fly. The second map shifts the scenery to the brain, where synapses form the new tracks. Here, trains represent nerve impulses racing between neurons, and the challenge is to deliver neurotransmitters before the synapse “fires” again. Because the mechanics are tightly coupled to the base game’s engine, the expansion is best suited for players already comfortable with Age of Steam’s pacing and strategy. It adds a fresh thematic layer while keeping the core pick‑up‑and‑deliver loop intact. Components include two new map boards, a set of extra track tiles, a few special cargo tokens, and a synapse marker that tracks the brain’s firing cycle. Playtime stays in the two‑hour window, and the player count remains 3‑5. The learning curve is moderate: you’ll need to read the new rules, but once you grasp the color‑change mechanic and the synapse timer, the expansion feels like a natural extension of the base game. It’s a great way to spice up regular Age of Steam sessions, especially for groups that enjoy a science‑fiction twist and aren’t afraid of a slightly out‑of‑the‑ordinary thematic layer.