For those who are still interested in wire height and the "visual pollution" of heavy rail overhead rather than badinage & discipline, here are more references. Caltrain will be a modern AC main-line electrification which, even if clearances are shrunk a bit due to lack of high-wide freight traffic on the Peninsula, don't work with traditional trolley/interurban rules; which is why, as Al Stangenberger so aptly put it, it's complicated. Plus, presumably, if HSR ever arrives to share tracks it'll need heavy-electric wiring to handle its power and clearance needs.

For more information, suggest looking at Caltrain's electrification project page: [www.caltrain.com]
Many questions will be resolved there and in the various documents linked from it.

Amtrak NE Corridor: about the only currently operating true main-line high speed electrification in the US.
Amtrak blog with a picture: [blog.amtrak.com]
Wikipedia on the NEC 25kv/25Hz electrification (dating from Pennsylvania RR days): [en.wikipedia.org]
Wikipedia on the NED 25kv/60Hz electrification (New Haven-Boston): [en.wikipedia.org]
Railpictures photo near New Haven; the strong telephoto shot makes it look more cluttered than it is: [www.railpictures.net]
Somewhat more "normal" Railpictures shot - less telephoto: [www.railpictures.net]
And a NJ-related document: [nec.amtrak.com]

METRA Electric District & South Shore: the only other main-line electrification in the US using overhead wire that I've aware of at the present time (some bits of the old NYC and the Long Island use 3rd rail). They use 1500VDC (more like an interurban, which the South Shore was) so the wiring differs considerably from a heavy-electric arrangement like Amtrak's.
Wikipedia on METRA Electric: [en.wikipedia.org]
WIkipedia on South Shore line: [en.wikipedia.org]
Railpictures - note clearance when diesel bilevel commute set was used in a no-electricity emergency: [www.railpictures.net]

High Speed Rail Things
Since CAHSR is, in part, paying for the Caltrain electrification and intends to share Caltrain tracks at some point in the future, their standards for overhead wire construction will probably be considered. See (PDF file): [www.hsr.ca.gov]
PUC Proposed 25KV General Order (PDF): [www.tillier.net]
PUC Pantograph Clearance Envelope (PDF): [www.tillier.net]
Indian HSR Design Document - Power System (PDF): [khsrcl.com]
Chinese HSR - somewhat distant shot: [i879.photobucket.com]
Dutch? [i93.photobucket.com]

There's probably lots more out there, but this should be ample to get an idea of what Caltrain catenary should be like.