Quick answer: Blue Tower progression is tied to the railway. Treat the route as a navigation project: collect rewards, prepare the key and learn the stations as a group.
What Blue Tower unlocks
The train network makes later exploration and cleanup easier. It becomes far more useful once the group agrees on station names and map landmarks.
Route checklist
Bring the required rewards to the Blue Tower; complete the indicated key route; keep the progression item with one player; use the finished key at the railway destination.
After trains start
Take one complete ride before splitting up; use stations as rendezvous points; pair train stops with nearby skipped puzzles and landmarks.
Source & update note
Core platform, player-count, crossplay and hosting facts are checked against the official Big Walk FAQ. This page uses spoiler-aware community route notes and should be rechecked after game updates.