How do trolley buses, such as the type in San Francisco or Seattle
that use two poles to connect to two wires strung above, switch at
junctions of sets of routes? Specifically, I'm curious how the
switching mechanism knows which bus route the bus is following and
flips the switch to the correct route.

Hello flahertyjp,

I believe that this page provides a sufficient explanation:

"Electric vehicle technologies," by James Strickland [section on "The trolleybus"]
Transport 2000 BC
http://www.vcn.bc.ca/t2000bc/learning/etb/electric_vehicles.html

This dictionary of terms is also useful:

Google cache of "Dictionary for Overhead Contact Systems with
Pantograph and Trolley Pole Operations" (Draft July 2004)
IIEE Rail Transit Vehicle Interface Standards Committee
http://64.233.167.104/search?q=cache:giPrSSx3ao0J:www.railstandards.com/ocs/IEEEOCSDICTIONARYRev3.pdf

Here is an alternative system:

"Inductive Control System (ICS)"
IMPulse NC, Inc.
http://www.impulsenc.com/pdf/ics_brochure.pdf

- justaskscott


Search strategy --

Searched on Google for various combinations of these terms:

trolley
"trolley bus"
trolleybus
switch
faq
"trolley poles"
poles
turn
selectric
"power on"

Thanks. The first link was exactly what I was looking for. It has the
three switching mechanisms commonly used and they are activated.

They don?t have to, the driver knows where he is going, the overhead
power lines are always live with a short gap or insulator at
junctions, the pickup rides over the gap or insulated section and
picks up on the next stretch of overhead power line.