07 December 2008

Headspans and Poles, Oh My!

The High Speed Rail and Caltrain electrification projects converge on one requirement: electrification with 25 kV overhead wires. This is a standard choice around the world, easy to implement, compatible with most train types, commercial off-the-shelf, and able to handle the high power loads (many megawatts) drawn by fast trains. The photo at right is a typical sample. (credit: Vitó) Electrification is a no-brainer, so you wouldn't think it could be screwed up.

Documents from the California High Speed Rail Authority and Caltrain show one particular way to electrify a four-track railroad. As Richard M. pointed out in a comment in another post, it is not the only way and it is certainly not the best way for the local conditions on the peninsula. Read on to understand why. (Warning: train geek alert. Proceed with caution.)

Supporting the overhead wires (also known as catenary) on a 4-track electrified railroad can be done with poles, headspans or gantries. The three options are described below.
  • Poles (a.k.a stanchions in British parlance) are placed between pairs of tracks, and have support brackets on each side to support the catenary wires. In a four-track arrangement, the poles are placed between the inside and outside pair of tracks.
  • Headspans are networks of steel cables hung across all four tracks, rigged from a tall pole on each side of the outside tracks. Mechanically speaking, this is somewhat analogous to a suspension bridge across the tracks. The catenary wires for each track are hung from the headspan wiring.
  • Gantries are rigid metallic portals that span across all four tracks. Brackets are hung from the horizontal member to support the catenary wires.
Each of these options has pros and cons. We'll discuss headspans first, since that is the option seemingly favored in the California High Speed Rail Authority's Bay Area EIR/EIS cross sections (Volume 2, Appendix E, Figure CC-8), as well as in the Caltrain electrification EA/Draft EIR (Chapter 2, Figure 2.3-3).


A headspan is shown in the diagram at right. The dimensions shown reflect the narrowest practical 4-track headspan arrangement that complies with Federal Railroad Administration and California Public Utilities Commission requirements (assuming those requirements will not be waived). While their thin cables are somewhat easier on the eyes than other options, headspans also have several significant drawbacks.
  1. Headspans are more complicated to maintain, since the headspan cables mechanically link the overhead contact system for all four tracks. Tweaking one cable may knock another cable out of alignment; replacing an electrical isolator on one track also affects other tracks, which may need to be taken out of service. On a busy 4-track railroad, this is not desirable.

  2. Headspans are more vulnerable to pantograph failures. Rarely, pantographs (the spring-loaded metal frames on top of trains that pick up electricity from the overhead wire) fail or snag on the wiring. While there are safeguards to limit the damage from such an occurrence, the damage can be quite extensive. With a headspan configuration, a failed pantograph can damage all four tracks at once, shutting down service entirely; with a bracket support, the damage is contained on one track. This video, showing a spectacular pantograph failure, illustrates the potential problem.

  3. Headspans require very tall poles located on the outside edge of the right of way. Not only is this ugly because taller poles dwarf surrounding structures and vegetation, but it requires trees to be trimmed back further from the tracks.

  4. High voltage (50 kV) feeder wires, strung from the top of those poles, are required by the CPUC to have a minimum of 4 feet of radial clearance (General Order 95, Rule 35, Appendix E). This 4-foot high voltage keep-out zone is shaded in pink in the diagram above. On the peninsula, in places like Atherton, Burlingame or Palo Alto where large trees sometimes grow near the tracks, the outside poles and feeders that come with headspans might require more heritage trees to be cut down to build HSR.

  5. Headspans are not easily reconfigured to add tracks. They need to be built to their full width from the get-go.
Given that headspans have these drawbacks, it's worth looking at the other options.


Gantry frames are aesthetically the most upsetting, as the photo at right shows. (credit: polandeze) This photo is sure to be a hit with detractors of HSR on the peninsula. Gantries are not just uglier than poles because of their massive horizontal beams, but they are structural overkill in the benign conditions of the Bay Area. Gantries are typically used in situations were mechanical loads on the wires are high, horizontal spans are very wide, or vertical clearances are limited. This is exemplified by some areas of Amtrak's Northeast Corridor between New Haven, CT and Boston, MA, where the relatively recent overhead electrification is overbuilt to withstand large forces from hurricane winds and the heavy buildup of ice during winter storms. Obviously, we don't need to worry about ice storms or hurricanes here on the peninsula, and we hope the HSR and Caltrain folks won't unquestioningly emulate Amtrak.


Poles located between the inner and outer pair of tracks, as shown in the diagram at right, have many advantages:
  1. Poles and brackets are easier to maintain without affecting multiple tracks

  2. Poles and brackets are mechanically robust to pantograph failures, containing damage to the affected track (as seen in the video above)

  3. Poles are much lower than headspans (32 ft above rail versus 43 ft, according to Caltrain engineering drawings) and therefore less visually obtrusive

  4. Poles keep high voltage away from the edges of the right of way, where they might interfere with surrounding objects and vegetation. The 50kV feeders are now hung above the tracks. As before, the diagram shows a pink 4-foot voltage keep-out zone, which is smaller and concentrated over the tracks, unlike headspans.

  5. Poles make it easier to build two tracks first, then add another set of outside brackets (but no additional poles) to accommodate four tracks without tearing out any of the existing electrification. This would add flexibility to the construction phasing between HSR and Caltrain electrification, enabling Caltrain to future-proof anything they might build before HSR.
Poles do have one slight downside: the California Public Utilities Commission, which issued the existing clearance standards for rail infrastructure in California in 1948 (that's right, nineteen forty-eight!), requires 8'3" clearance (2.51 m) between a pole and the center line of the track. Including 1'6" for the tensioner assemblies that hang from certain poles, that puts the minimum track spacing at 18 feet, about 3 feet more than without the poles. Nevertheless, overall width of the right of way may not increase that much because less clearance is required along the edges, where poles are not present.

For either poles or headspans, the four-track electrified right of way running at ground level can fit within 70 feet (21.3 m), in a pinch. While the foregoing discussion is somewhat arcane, it will be quite relevant in those situations along the peninsula where the Caltrain right of way is narrowest, where HSR may cause greater community impacts and possibly eminent domain takes.


  1. Good morning,

    Thanks for another great article.

    I am in over my head here, but what about a third rail and paddle like BART? Just on the peninsula. The ROW is fully separated. The speeds will be the lowest on the system, but maybe not low enough? Huge improvement for the backyards. Correct me if I am wrong but Eurostar used a retractable paddle in England when it first opened.

    Thanks again,

  2. Catenary from Wiki.


    "In physics and geometry, the catenary is the theoretical shape of a hanging flexible chain or cable when supported at its ends and acted upon by a uniform gravitational force (its own weight) and in equilibrium. The curve has a U shape that is similar in appearance to the parabola, though it is a different curve."

    Also as you build a suspension bridge you string a catenary cable and as you add the load of the deck structure it pulls the cable into a parabola shape.

  3. Anon -

    I'm over my head as well, but one of the main points to the CAHSR concept is to keep it standard. I'd be concerned about creating a "unique" system that we can no longer buy cheaper, off-the-shelf components for...

  4. @Anon,

    Third rail is close to the ground, runs at low voltage (in the ballpark of 1000 V) and therefore requires much higher current. Drawing enough juice from a 3rd rail to power fast trains (rated at roughly 10 megawatts) is technically infeasible.

    Eurostar HST's did once use 3rd rail, but they could only develop about one quarter of their full rated power due to the above-mentioned limitation. That made their performance (speed, acceleration) positively anemic. That got fixed with the opening of the Channel Tunnel Rail Link, electrified with standard 25 kV overhead.

  5. @ Clem -

    the poles would indeed be the least obtrusive and possibly also the cheapest solution. I'm not sure if any of these designs is more resilient in earthquakes than the others or, if that even matters.

    Here are some catenary pole designs from around the world:

    - Austria
    - Belgium
    - Czech Republic
    - France
    - Germany regular
    - Germany HSR
    - Italy
    - Japan
    - Netherlands
    - Portugal
    - Switzerland
    - Taiwan
    - UK
    - USA (NEC)
    - various 1
    - various 2

    Note that the two center tracks require greater horizontal separation than usual in US railroad engineering due to relative speeds in the 250-300mph range. You don't want the bow waves to blow out the windows.

    @ anon -

    third rail would be possible in principle, since all four tracks will be fully grade separated anyhow. However, third rail operates at lower voltage (e.g. BART at 1500 VDC), which means power is constrained by the electrification system. In addition, every single trainset would have to specially adapted with a third rail pickup and the associated power converted. It's a much better idea to stick with standard overhead catenaries at 25kV AV single phase @ 60Hz.

  6. @ Clem

    Is the third rail that inferior to the overhead centenary? Is BART suffering from this inefficiency due to lower voltage? Is it too much to hope that BART could be upgraded to 25KV overhead power? I expect the tunnels are not tall enough to even consider overhead. It would be a major upgrade, but there is time. After the BART infrastructure serves a lifetime maybe it could be replaced with better rails, power supply, and rolling stock. Then go ahead and make it standard gauge. Hmmm.

  7. @James - BART is a different cup of tea, and it's just fine the way it is. It is both much lighter (by about a factor of 2, on a per-seat basis) and much slower (80 mph max) than HSR. 3rd rail can't provide enough juice to accelerate 800 tons of train to 100 - 150 mph, let alone 220 mph. 3rd rail would *severely* cripple HSR and Caltrain, and put them out of step with the de-facto world standard. It would complicate off-the-shelf acquisition and blow budgets and schedules sky high. We really, really don't want to go there.

    @Rafael - 15'4" isn't a lot more than the 14' US minimum (which is itself very ample, same as LGV Sud-Est if that means anything to you)

  8. I believe there are also contact issues with 3rd rails as you exceed 100 mph. 3rd rail is good for subway/rapid transit because it is more durable/reliable than overhead catenary (which can, as Clem points out, get snagged). But it's not a good choice for HSR.

    1. There certainly are contact issues at high speeds.

      The largest region of 3rd rail, and one of the very few places that it's used for regional (as opposed to metro) service is the former Southern Railway region in England, where the longest run is about 100 miles (London Waterloo-Weymouth). There was extensive testing in the 1980s, to try to find the highest speeds that could reasonably be attained with this transmission system, and the record run was 108mph - still the world record for third rail. The highest speed used in service is 100mph. The power contacts cannot reliably operate at higher speeds.

      There are also power issues - Third rail has to be much lower voltages, typically 750V to 1500V (and is usually DC rather than AC) because of the risk of discharge. This reduces the amount of power available.

      On the other hand, third rail has less aesthetic impact than overhead, is physically smaller (meaning that tunnels can be smaller and therefore cheaper) and is more easily compatible with existing infrastructure (ie if you are electrifying an existing line, you don't need to raise bridges to fit the wires underneath).

      For cost and safety reasons, overhead is generally preferred unless there is an existing third rail system to be compatible to, or unless it's an entirely-underground-tunnel metro system.

  9. Re: Overhead images.

    I look forward to US passenger rail catching up with Portugal and Czech systems. The US needs to wake up and join the rest of the world. We have focused too much for too long on the Cold War and the automobile and have been left behind. To some extent the US investment in the Cold War left other countries free to invest in rail infrastructure. Nothing like a swift kick to get the US moving.

  10. @ Clem -

    CHSRA wants to use UIC-compliant off-the-shelf bullet trains from Europe or Asia. for example, a fully equipped 14-car Alstom AGV with 650 seats and 12,000kW worth of traction motors weighs 510 metric tonnes - not 800.

    It's only the FRA in the US that forces passenger trains to be built like sherman tanks. Avoiding the Acela Express debacle is a big part of why CHSRA decided to build all-new tracks wherever it is at all possible to do. In the few locations where it isn't, guaranteed time separation between FRA-compliant and non-compliant equipment is required.

    Basically, FRA treats anything that does not conform it its own inferior crash safety regulations (see Appendix C) as if it were toxic waste, unless it runs on its own tracks. BART equipment is also non-compliant.

  11. The MUNI's trolleybuses have poles that retract as soon as they lose contact with the overhead. Ugly messes still occur, but much less often.

    I've seen poles hook the overhead and rip down the energized lines into the street, with sparks and all. I assume there is some sort of system like this for railway pantographs.

  12. OK, poor syntax.

    I assume there is a system that retracts HSR pantographs when something happens, not a system to make HSR pantographs rip down overhead.

    It's the result of a lifetime of California public education, most post-Prop 13.

  13. Agreed. Campaign for poles. :-)

  14. German (EBO regulatory) standards for pole clearance:

    * Two tracks > 160kmh flanking central masts: track centres 6.8m.
    * Two tracks, one <= 160kmh, one > 160kmh: 6.4m.

    Oh yes, clearance from > 160kmh track to any adjacent wall or railing: 3.8m. (3,3m <= 160kmh).
    (Compare 4.3m shown in EIR!)

    (These numbers are all based on a 3.0/2.5m danger zone and a 0.8m safety refuge zone.)

    On the other hand the uniform 4.7m track spacing CHSRA show is wasteful and unprecedented, and can only be explained by Advanced American Engineers -- like those who lost the Maris Climate Orbiter -- pulling a figure like 15'6" out of the air and deciding it was a good number for something or other -- perhaps grain silos sidings in Nebraska? How do they dream this stuff up?

    Anyway, the German new construction standard for plain track spacing is 4.0m for <= 200kmh, 4.5m for > 200. Shinkansen centres are 4.3m.

    We can pretend that CHSRA consultants knew what they were doing about when they sketched out the typical cross-sections in the EIR, but I wouldn't bet on it, and I wouldn't bet anything at all on US regulators allowing tighter clearances, smaller safety zones, and less evacuation space than existing and well-regarded foreign systems.

    The cross-sections are only going to get wider and the impacts larger, and the costs greater, in other words.

    So, rather than start with CHSRA's sections -- which like its financial projections, run-time estimates, service plans, and environmental assessments seem just a little ... off -- I'd recommend going with real-world engineering standards from real-world, well-regarded and proven competent organizations. (ie The ones whose workers' first language isn't English.)

  15. Clem,

    I noticed that there are very few HSR lines in the world using one pole to support catenaries over two tracks. The most common cases are one pole supporting one track, and for areas there are more than a pair of tracks, it is either supported with gantries or 4 poles.

    For station areas, I don't think gantries would be an issue, as many HSR station structure had Catenary support built into their super structure.