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).

Headspans

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.

Gantries

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

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.