The Short Answer
Clear the Blue Tower — the fourth and final tower in the recommended Red → Green → Yellow → Blue order. Finishing it unlocks the Monorail train system, which is the last of the four tower travel rewards.
What the Train Actually Is
The world features three-coloured trains — red, yellow and green — that correspond to three coloured stations ("red train, yellow train, green train"). They're a primary way to move around the island once available.
There's a known open question: sources describe both a "Monorail" (the Blue Tower unlock) and separate red/yellow/green trains seen in gameplay. These may be the same system viewed differently, or two distinct travel methods. This is to be confirmed via a full playthrough video and is not yet resolved.
Why Save Blue for Last
The same community guide that ranks the tunnels as fastest calls the monorail slow and infrequent ("pretty unhelpful"). That's exactly why the recommended order does Blue last: by the time you've got the map, chairlift and tunnels, the train is a nice-to-have rather than a necessity.
Working With the Other Systems
- Tunnels (Yellow Tower) — fastest point-to-point travel.
- Cable Car (Green Tower) — required for the east-ridge challenge area.
- Shortcuts — missable bonus routes.
- Train / Monorail (Blue Tower) — scenic, slower, but completes the network.
Still being documented: the coloured station locations, the exact drive / board method, and whether the monorail and the three-coloured trains are one system. This page reflects the verified unlock (Blue Tower → Monorail) and flags the open conflict honestly rather than guessing.