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The Portland Aerial Tram. Image by functoruser at flickr.

Aerial Trams are the granddaddies of cable transit. They’re big, they’re aggressive and what they do, they do really well. Problem is, they can’t do much. They’re a completely antiquated technology due to their lack of detachability.

Like BDG or 3S systems, Aerial Trams use one or two stationary ropes for support while a second or third moving rope provides the propulsion. But unlike BDG and 3S systems the Aerial Tram’s grip is fixed and cannot be decoupled from the propulsion rope during operations. This means that corners are all but impossible in an Aerial Tram configuration and intermediary stations are limited to single mid-points along the line. These mid-stations are incredibly rare.

This has created confusion amongst many. I’ve seen high-level research reports that have stated unequivocally that cable transit systems are not capable of turning corners. This is no doubt due to a confusion between Gondola and Aerial Tram technology. Problem is, the fact that Aerial Trams cannot turn corners gets extrapolated to mean that all cable transit cannot turn corners, a fact which is demonstrably false. So let’s set the record straight:

In almost all cable transit systems, corners and intermediary stations are simple to implement. The lone exception is with an Aerial Tram whereby corners and intermediary stations are all but impossible. Do not make the mistake of assuming that the specific limitations of Aerial Trams equate to a limitation of cable technology in general.

A simplified diagram comparing how detachable Gondolas and Aerial Trams operate: (Top) Small vehicles in a gondola system constantly circulate. (Bottom) Larger Aerial Tram vehicles shuttle back and forth.

This lack of detachability points to another failing of Aerial Tram technology. Because vehicles cannot circulate throughout a system as in a Gondola-type situation, vehicles must shuttle back-and-forth. This means that only two vehicles are possible and each vehicle is dependent upon the other. Not until each vehicle is ready to depart can either vehicle move. The only solution to this problem is a Dual Shuttle configuration that uses two separate rope loops which allow each vehicle to move independently of the other:

In a Dual Shuttle configuration, Aerial Tram vehicles can operate independently of one another. This adds additional cost to the system, but improves capacity and operational benefits.

This solution increases the cost of the system, but provides a marginal increase in capacity and decreases wait times. It also would allow for intermediary stations as each vehicle operates independently. Furthermore, a Dual Shuttle allows the system to operate 24 hours per day as one line can be taken out of service at a time for maintenance while the other continues to operate. Similarly, in the event of the mechanical failure of one line, the other line can still be operational.

The shuttle-based nature of Aerial Trams means that capacity and wait times are directly proportional to the length of the system. The longer the system, the longer the wait times between vehicles and ultimately, the lower the capacity. Because of these limitations, Aerial Trams compensate with incredibly large cabins of up to 200 people, by far the largest of all aerial cable transit systems.

This compensation only goes so far, however. Aerial Trams can really only move 2,000 pphpd and such capacity would be possible only in the shortest of systems. Detachable gondola systems, however, can move up to 6,000.

Ironically, despite having the greatest number of limitations, Aerial Trams are some of the most expensive cable technologies around. This owes to the massive size of the infrastructure needed to carry three cables as well as increased station sizes.

Aerial Tram Stats:

• Maximum Speed: 45 km/hr.
• Maximum System Capacity: 2,000 pphpd.
• Vehicle Capacity: Up to 200.
• Capital Cost: Approximately $10 – 50 million (USD) / kilometre.

So when then should you choose an Aerial Tram over other cable transit systems? No where. At least none that I can think of. There is no benefit to Aerial Trams that other cable systems don’t also possess without the large number of limitations that come with Aerial Trams.

The one potential benefit of Aerial Trams I can see is that of privacy concerns and visual pollution. Rather than seeing a small gondola fly overhead every 10-30 seconds, one sees a single vehicle every 5-10 minutes. That alone, in my opinion is the only benefit of the technology but is offset by the overwhelming number of limitations.

The advent of detachable systems rendered Aerial Trams obsolete, but people have held onto them due to some bizarre connection to the past. For example, the $25 million rebuild of the Roosevelt Island Tram (which is going on right now) is replacing the old Aerial Tram with a new Dual Shuttle configuration, a definite improvement over the original.

Replacing the Roosevelt Island Tram with a gondola system, however, would have provided a greater level of service at a cheaper price. As per my understanding, that alternative was never considered, likely because of the Tram’s iconic status. Were the Roosevelt Island link conceived and built today, it likely would’ve been a 3S system.

The debate that’s going on right now about Aerial Trams is this: Is it technologically feasible to combine the high-capacity vehicles of an Aerial Tram with the detachable capabilities of other systems? If so, a dramatic increase in system capacity could be realized. Such an increase could allow aerial cable systems to carry passenger loads approaching that of subways and metros but at a quarter of the cost.

That would be a game changer the likes of which the cable industry has never seen.

Proceed to Aerial Technologies, Lesson 6: Pulsed Gondolas

Return to Aerial Technologies, Lesson 4: Funitels