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.

Portals

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.