Last weekend, I tweeted a few photos of Expo Line Phase 2 overhead contact system (OCS) construction. The OCS is part of the traction electrification system (TES) that provides electricity to trains, and includes the poles, cantilevers, wires, and associated hardware that you see along the track. The other part of the TES is the traction power system (TPS) which consists of electrical substations along the tracks, the cables that connect the substations to the OCS, and the rails, which serve as the negative return.
In this post, we’ll explore some OCS choices that can impact the cost of a light rail project. The OCS is something you have to build, and it’s not going to be the thing that breaks the bank. Nevertheless, a million here, a million there is still real money, right? In the same way that Jarrett Walker describes the choices between frequency and coverage, I’m going to take the soft approach and tell you that none of these options are really wrong, but you need to have an honest and open discussion about the cost implications. A good engineer should provide the owner with an understanding of the options available and the implications of each, and faithfully execute the design as efficiently as possible, but the ultimate design direction is made by the owner.
The discussion here is geared towards DC electrification, which is used for most light rail systems. Intercity rail is electrified with AC; the concepts are the same, but the greater working clearances required for higher voltages will result in some different decisions (e.g. side poles instead of center poles).
System Type and Height: Normal Profile Simple Catenary, Low Profile Simple Catenary, or Single Wire?
Simple catenary refers to the system you see on most of the Expo Line, Blue Line, Gold Line, and Green Lines. This system has two wires, called the messenger wire and contact wire. The messenger wire is the upper wire and is more visibly parabola-shaped than the contact wire, which, as the name suggests, is the wire the train’s pantograph touches. The system height refers to the vertical distance between the messenger wire and the contact wire at the poles.
A normal profile simple catenary system might have a system height of 4’. This lets you maximize the distance between poles (the span length) based on other factors like wind loading and track curvature, without worrying about maintaining separation between the messenger wire and the contact wire. So why aren’t all systems normal profile simple catenary? Because people have decided they’re visually unappealing.
A normal profile simple catenary system takes up a lot of your field of vision and has long hangers connecting the messenger wire to the contact wire. To reduce the visual impact, you can use low profile simple catenary. This has a lower system height – say 2’. This reduces the visual impact of the wires, but because the messenger wire and contact wire start out closer at the poles, they’ll get close to each other sooner, and you can’t space the poles as far apart.
Thank goodness the OCS doesn’t block too much of the view of the LADWP power lines on the other side of the tracks, right?
As an example of pole spacing, here’s a straightaway on normal profile OCS on the Blue Line with poles at 180’, and one on the Green Line with poles at 220’. Meanwhile, the straightaways on the Expo Line’s low profile OCS max out at about 140’.
The main impact here as far as cost goes is that you need more poles, and pole foundations. As you can see, the comparison between the Green Line and Expo Line suggests a low profile system will need three poles for every two poles on normal profile system. That’s too high, because curves will still need closer pole spacing, as will special track work like crossovers. Nevertheless, using a low profile system can result in a considerable increase in OCS costs. The engineer should be able to give a rough idea of the impact of changing the system height for the project. The owner must decide if it’s worth the cost.
Finally, there is the single wire system. This is often the most preferred system by politicians and city boosters because it results in the fewest wires in the sky.
Naturally, single wire is also often the most expensive. The pole spacing is reduced because without the messenger wire, you need more supports to keep the contact wire on an acceptable profile. However, the hidden costs of single wire systems are worse. With only one wire in the air carrying current, there is not enough ampacity (ability to carry electricity) in the system. To avoid the need for additional substations, power must be supplied to the contact wire at shorter intervals, and this is done with electrical feeders in underground duct banks. This can add significant costs.
Single wire systems are most suitable for low speed services in touristy areas, which is why you often see them for streetcars. They’re also suitable for tunnels and other places with constrained vertical clearances, where the cost of feeder cables and additional supports is less than the cost of increasing the vertical clearance to accommodate a simple catenary system.
Poles: Wide-Flange, Tubular, or Ornamental?
This is again strictly an aesthetic decision. Wide-flange poles are, as the name suggests, wide-flange steel beams, the kind you can practically order off the shelf. That makes them cheap, because you just say you need so many W this by that members of lengths X, Y, and Z. They’re also very utilitarian and you rarely see them in urban contexts. Here they are in action on the Northeast Corridor, where it would be hard for anything to outdo the New Haven’s old rusty trusses with outside utility overbuild.
Tubular poles (or tapered tubular poles) are a little bit more pleasing to the eye, probably because they tend to look like the poles used on old systems. They’re a little more expensive, but not terribly: this Central Corridor document suggests that the costs to go from wide-flange to tubular poles for 5 miles of ROW was just $1m. Of course, if you have chosen a low-profile simple catenary, you’ll have more poles, and the cost will be greater.
Ornamental poles will cost you quite a bit more, like any custom design. Here are a couple examples, one in Australia and one in St Petersburg (the Leningrad one, not the one that had a Madame Tussaud Wax Museum from 1963 to 1989).
A couple of final points: wood poles are always an option; I’m not aware of any modern system that’s using them but they are cheap. Of course, they’ll probably wear out faster than steel poles. The built-up truss style poles you see on older systems are labor-intensive to manufacture, and standard wide-flange steel beams are widely available. If material costs for steel go up, maybe pre-fabricated truss poles like these ones in Australia will be more cost-effective. (The source page for that photo has other good photos of wide-flange poles too.)
Another potential costs savings in OCS design is using aluminum for the messenger wire instead of copper. Aluminum has a lower ampacity, which means you have to use a slightly larger gauge wire, but this is more than offset by the cheaper cost of aluminum. Aluminum conductors are sometimes used in building construction to save money; for example, the Staples Center used aluminum feeder cables. On the other hand, this paper (registration required) recommends the use of copper to avoid the need to connect different metals electrically, which can create a galvanic couple. This page from the Indian Railways Fan Club says that Indian Rail tried aluminum contact wires but this was unsuccessful due to oxidation and mechanical failures. I’m not sure why they tried aluminum for the contact wire; copper is definitely the way to go there.
My knowledge here is very limited, but I’ve heard that there are places outside the US that use aluminum for the messenger wire and copper for the contact wire. Certainly, this is an option worth considering if the price of copper stays as high as it has since it surged before the 2008 financial crisis.
The OCS is just a cost of doing business, and it’s usually not a large portion of the total costs. The big overruns come from unnecessary project elements, labor inefficiencies, differing site conditions and associated delays, change orders, and so on. But, while the cost of a house is mostly determined by the size of the house, a lot of little extravagances with the finishes can noticeably drive up the costs too. Do you really need marble floors in the bathroom?
As an engineer, I usually come down on the side of cost efficiency. If it was up to me, you’d get nothing but standard wide-flange poles and normal profile simple catenary. No one’s life is going to be measurably worse from having to look at those poles with two wires, and no one’s life is going to be measurably better by getting to look at an ornamental pole with one wire – certainly not in comparison to having or lacking access to good transit. At least, that’s my opinion. If your city decides it’s better to have fancier looking things, that’s your collective choice. Just have an open and honest discussion about the costs.