Riverfall is the largest and most straightforward route from the surface to The Machine, made somewhat more challenging by the river that gave it the name. When the empire discovered that the immense caves above The Machine were packed to the brim with metals, coal, and oil, they went nuts. The empire needs nothing as much as metals, coal, and oil, especially from a source so near their homeland and in such rich quantities.
The city beneath the shaft has almost ten thousand miners inhabiting it. They call it the "Machine City". The shaft itself is inhabited by a lattice of almost-verticle cargo gondolas, most of which work at any given time. Those which are not working are supplanted by fleets of cargo blimps. The chain is used both to send ores to the surface and to receive food, cargo, and people on return.
The whole city - and each gondola station - is supported by titanic water turbines powered by the crashing weight of an entire river. Since electricity has never been discovered, everything is mechanically powered by an immensely complex series of line-driven machines.
All the lights are phosporescent globes. They last about a week before they need to be re-injected with the chemicals that make them glow. They only come in two colors: blue and a vastly more expensive orange.
Riverfall is the only "official" incursion into The Machine. Other routes (and free operators) were banned since so few returned from them. Even the Machine City has a hard time of it - not only are the caves filled with physical dangers - cave-ins, gas pockets, monsters - but also with mysterious psychological dangers.
You and your team have no connection to the Machine City or Riverfall.
Early concept sketch of Riverfall: