05 April 2014

Saving Some Trees

One of the most controversial elements of the Caltrain electrification DEIR is the need for tree pruning or removal to establish adequate clearance between vegetation and the live parts of the overhead contact system, energized at 25,000 volts or more.  While the exact regulatory details governing 25 kV electrification are still being hammered out at the CPUC, vegetation clearances are likely to be governed by General Order 95, Rule 35, Appendix E, which states:
The radial clearances shown below are recommended minimum clearances that should be established, at time of trimming, between the vegetation and the energized conductors and associated live parts where practicable. Reasonable vegetation management practices may make it advantageous for the purposes of public safety or service reliability to obtain greater clearances than those listed below to ensure compliance until the next scheduled maintenance. Each utility may determine and apply additional appropriate clearances beyond clearances listed below, which take into consideration various factors, including: line operating voltage, length of span, line sag, planned maintenance cycles, location of vegetation within the span, species type, experience with particular species, vegetation growth rate and characteristics, vegetation management standards and best practices, local climate, elevation, fire risk, and vegetation trimming requirements that are applicable to State Responsibility Area lands pursuant to Public Resource Code Sections 4102 and 4293.
10-foot tree clearances, from DEIR
The CPUC rule establishes minimum clearance of 4 feet at the time of trimming, and gives Caltrain discretion to increase the clearance as needed.  Caltrain has added another 3 feet of allowance for wind sway, and another 3 feet on top of that for vegetation growth between trimmings.  This brings the overall clearance carried in Caltrain's DEIR to 10 feet, as shown in the diagram at right.

This 10-foot clearance drives the number of trees impacted by the project: 2,200 are due for removal and another 3,600 for pruning according to the DEIR.

Outside Poles

The standard configuration of the overhead contact system places poles on the outside of the tracks, near vegetation.  The clearances look like this:
This configuration tends to have the worst impact on vegetation, mostly because the 50 kV feeder wires attached to the tops of the poles are near vegetation all along the right-of-way, even in the long open spaces between poles (right-hand diagram).  This is the main reason why 2,200 trees face the chain saw.

Inside Poles

To keep high-voltage components away from vegetation, it is possible to locate the poles between the tracks in the middle of the right-of-way.  This requires slightly more space between the tracks; the regulatory minimum is approximately 18 feet (2x 8'3" minimum from track center line to pole face, plus the width of the pole itself, plus some error margin).  The diagram below shows 19-foot spacing, with taller poles to carry both feeders with adequate clearance from each other:
While this configuration spreads the tracks apart by four feet, the resulting impact to vegetation is less, especially between pole locations, because the feeders are kept away from vegetation.  The down side is that tracks need to be moved apart; this is not very difficult except where cross-overs are located.


Portal gantries are basically the same as side poles, with a cross-bar across the top.  Like center poles, the portal arrangement allows the 50 kV feeders to be located in the middle away from vegetation.  Unlike center poles, portals do not require the tracks to be further apart than 15 feet:
The portal configuration has the least impact on vegetation between poles (right-hand diagram) but isn't particularly beautiful (left-hand diagram).

Most of the 2,200 trees threatened by electrification probably aren't worth saving.  They are typically not "heritage" trees, and consist mostly of unremarkable species that have grown haphazardly into the right-of-way.  But there are surely certain trees worth saving, and for those, the support arrangement of the overhead contact system can be engineered to keep high-voltage feeders away from vegetation, over the tracks.


  1. One alternative portal configuration has the live parts attached to a vertical pole descending from the middle of the crossbar and ending above the roofline of the train. This gives an electrical clearance profile like that of the "center pole" arrangement, while still allowing for a 15 foot track spacing. This sort of arrangement used to be fairly common on older (and DC) systems in places like the Netherlands, Belgium, and South Africa.

  2. Multi-track side cantilevers are another (not great) solution.

    etc etc etc.

    They seem to love them in Britain even for some mainline track, for a reason I don't understand, but elsewhere in the advanced civilized non-Anglophone world they're mostly special cases in constrained locations like station platforms and station throats.

    1. Check out figure 2-7 of the DEIR, showing the same thing. Except the feeders are still located on the extreme ends of the extra-wide, reach-into-the-trees crossbar! Makes you wonder what silly design rule is buried somewhere in the AREMA standard.

    2. Maybe I'm just a primitive Anglophone untermensch, but what's the problem with them, other than the support pole is a little taller than it otherwise would be?

      It seems like a good solution - the big disadvantage to center poles is that any maintenance or repairs to them are likely to disrupt both tracks; this gets around that problem and confines all the extra construction to one side.

    3. Clem, you're right, figure 2-7 really is quite glorious. I'm especially fond of how the insulators for the feeder wire for the track on the pole side stick out beyond the right of way. That seems truly award-winning.

    4. but what's the problem with them, other than the support pole is a little taller than it otherwise would be?
      (Tried to reply at length with nice citations and links and all but stupid unmaintained google orphan blogger.com ate everything, so try again more briefly)

      Yes, taller, which is a small visual impact.

      But the main issue is loss of mechanical and electrical independence of tracks. That an increased mechanical complexity.

      Simpler is better, on both of those fronts.

      Simple side poles on double track line mean that overhead can be taken out (either "taken out" in the mechanical tangled-up and torn-down sense, or in the scheduled taken out of service for maintenance sense) without affecting the adjacent track.

      (OK, I'm pretending this isn't America, with all the insane "flagging" rules and maximized inefficiency in anything main line rail related. But even here, it's possible this applies a little.)

      For four tracks there's enough redundancy that between-track poles with pairs of simple hangers on either side (or simple poles with simple hangers for each adjacent track independently, depending) aren't such a reliability issue.

      Lighter simple stanchions with simple hangers have simpler foundations and are easier to site that multi-track cantilevers supporting higher loads and larger bending moments. (And short poles attached to cantilevers with heavy structural steel cantilevers are heavier and experience larger side moments than taller poles with diagonal guys, so lose-lose trade-offs there too.)

      Basically, simpler is better. Simple standard interchangeable parts are better. Simpler maintenance is better. More exotic solutions (curved turnouts, overhead gantries and headspans, etc) only where simplicity has downsides.

  3. Atherton resident and long-time Caltrain/HSR critic: To Caltrain: Don't electrocute our trees

  4. Trees fall down. You don't want trees near your overhead wires. Especially invasive weed species trees.

    1. People like Jack Ringham and Morris Brown don't care about the trees (weedy or not) nearly as much as they just know that they don't want more trains -- especially HSR trains -- running through their neighborhood.

      So they've become very interested in advocating anything that undermines the need or rationale for electrification. Oh the trees! Why not "try" tier 4 diesel locomotives?! Hybrid locomotives! Batteries! LNG! Hydrogen! Di-lithium crystals!

    2. Ringham & Brown surely know that diesels won't cut the mustard where performance is concerned. I can't agree more about their call for reducing air pollution, but Tier 4 isn't the way to do it--and Tier 4 is only for pollutants other than CO2. In their scenario, all that CO2 is still belched into the atmosphere, unlike electric trains that can feed braking energy right back into the electric grid. The corny poetry was cute, it must surely have taken a college degree from somewhere prestigious.

  5. Does anybody know the relative cost differences of the shown pole configurations?

    1. I don't think that is a relevant question.

      Caltrain's consultant costs and "agency overhead" and work practices are so extra-terrestrially out of control that actual real world physical costs like steel and line-side real estate simply do not matter. They're not even rounding errors when there's 200% of extra cost going to ... negative achievement.

      Look back at San Bruno: $150 million (!!!!!!) for a botched and useless grade separation that is now a permanent service problem, one that could have easily have been solved with under $4 million of property acquisition.

      Cost doesn't matter.
      Service doesn't matter.
      Earmarked capital budgets matter.

      So, maybe there might be a one to five million dollars, or even, as a real outlier, 15 million in cost difference through keeping some lineside residential property owners quiet. But the "budget" for the project is over 1500 million, and as we know from similar undertakings, the sky is the limit for blowouts and changes and "unexpected conditions" once it gets under way.

      Also to think about: narrow places where jiggery-pokery with pole position might save one or two trees might be exactly the places where Caltrain, if it had any notion of a service plan, ought to be most aggressively building up to and even outside the existing right of way. Really bad precedents are possible. (But whom am I kidding except myself? Caltrain has never once provided any good precedents. Allow me stab myself repeatedly in the face now.)