Aerial Tramway

A big glass box climbs a mountainside on a rope you can see from a mile off. The trick is that a second, identical box is coming down the other way at the same instant — and each one is quietly holding the other up.

How it works

An aerial tramway (a "cable car" in the British sense, or reversible ropeway in the trade) hangs its cabins from one or two fixed track cables — thick, stationary steel ropes strung terminal to terminal that act as the rails. The cabin rolls along the track cable on a wheeled carriage. Propulsion comes from a separate, moving haul rope: a looped line, clamped permanently to the carriage, that a motor at one end winds in and pays out.

Because the cabins are fixed to the haul rope, they cannot be released to circulate. Instead they shuttle — run out to the far terminal, stop, and run back. Almost every aerial tramway carries exactly two cabins, one on each half of the loop, and they always move in opposite directions.

The counterbalance trick

Two cabins on one rope is not an accident of symmetry — it is the whole point. As one cabin climbs, the other descends, and the descending car's weight helps pull the climbing car up. On a purely gravity-driven "jig-back," a loaded down-car can haul an empty up-car with no motor at all. On a powered line the motor mostly fights friction and the imbalance between the two cabins, not their full dead weight — so a surprisingly modest drive can move a 150-passenger cabin up a steep grade.

Big spans, modest throughput

Fixed track cables can be tensioned far harder than a moving rope, so aerial tramways throw enormous unsupported spans across gorges, fjords and valleys — long stretches with no tower in between. That is why they show up over terrain nothing else can cross.

The cost is capacity. Only two cabins are ever in service, and each round trip includes loading, the run out, and the run back, so riders wait for the cabin to return. A gondola, with its stream of small detachable cabins circulating nonstop, moves far more people per hour; a tricable (3S) splits the difference — gondola-style circulation on tramway-grade fixed track cables. Reversible tramways make up some ground with speed, running up to roughly 10–12.5 m/s (about 36–45 km/h) on the straightaway.

• Fixed track cables carry the weight; a separate haul rope provides the pull.
• Two cabins, always paired — each counterbalances the other.
• Long spans, low frequency — great over gorges, weak on throughput.

Famous examples

The Roosevelt Island Tramway in New York — the first aerial tramway used for daily mass transit in North America — swings commuters over the East River beside the Queensboro Bridge. The Sandia Peak Tramway outside Albuquerque runs about 4.3 km up the Sandia Mountains, one of the longest reversible spans in the world. Hong Kong's Ngong Ping 360 carries visitors to the Tian Tan Buddha, and Bergen's Ulriken (Ulriksbanen) lifts you above the Norwegian fjord city to its highest peak. Different continents, same machine: two cabins, a pair of ropes, and gravity doing half the work.