Since the A-P forumn has had a lot of great SIGNAL topics lately, I thought I would bring up the subject of a VERY dense signaled stretch of track that was some FOUR miles in length, but was protected by "145" signals for SAFETY! We're talking about the double-track mainline of the "Key Route" during the days when the "Key Route Pier" was in full operation!

The stretch of track ran from the KEY ferry terminal, across the 16,500 foot trestle, and terminated in the vicinity of San Pablo Avenue near 40th Street. The mainline was shared by Transbay trains of the Key Route (Key System) as well as Interurban trains of the Sacramento Northern.

As I recall, Old Pole Burner's grandfather helped to install some of these devices ! Maybe he could shed some light on this installation!
In Bridge Railway days, GRS provided most of the Interlocking.

Cheers.

KRK