Getting deeper into the mines

The mines go down several levels, but their corridors don't seem to connect.
Nudge 1Where to look
Each level's parts are joined by a lift, and some floors have a gap to drop through.
What you needOther problems to solve first
Solve these first (each opens its own page, no answers on the way):
Show the solutionThe whole answer, with a video
From the upper corridor, ride the lift on the far left down into the left chamber, and drop through the gap in its floor onto the railway level. There, the lift in the middle joins the two corridors; the lower corridor has a drop at its left end into the deepest tunnels. Down there, one lift drops to the lowest tunnel and another, at its east end, climbs to a room high on the right.