In the past I have made reference to various "two aspect" cab signaling systems, specifically those used on the Illinois Central (since retired) and the former Chicago & North Western east-west main line, now operated by Union Pacific. Aside from the
cab signal display of current Union Pacific locomotives this two aspect system can be seen in the cabs of old METRA locomotives and
Illinois Central signal rule charts.
If a two aspect signaling system sounds bizarre you would be correct. It's a pretty fundamental principle that trains cannot stop on a dime and need some warning before encountering an occupied block. Well the two aspect cab signaling system (Clear and Restricting) adopted by both the IC and CNW worked a lot better than you might think. A
somewhat recent article on Carsten S. Lundsten's US signaling website goes into detail about how the system functioned on an IC single track Automatic Permissive Block segment in Iowa, but it is probably best summed up in this single .gif image.
The cab signal "code" (pretty much the presence of an AC current frequency or even a DC voltage), is transmitted from any signal point displaying more favourable than approach. Passing a yellow signals will cause the cab signal to drop to Restricting and the train will continue on at Restricted speed until the cab signal returns to Clear. To ease the confusion between wayside and cab signals, both the CNW and IC used this system without fixed wayside signals except at distants and interlockings.
Alright, now I'll bet you are just thinking this system is horribly inefficient as trains will be crawling at Restricted speed instead of a less odious 30mph Approach. The key consideration here is to remember the braking time and for most freight trains, getting slowed down from MAS to Restricting will take a good chunk of the Approach block. Let's see what the GCOR Rulebook has to say on this issue.
17.5.1: Over 40 MPH
The high speed whistle will sound when the speed is more than 40 MPH when the cab signal changes to a Restricting aspect.
- Move the brake valve handle to SUPPRESSION within 6 seconds to prevent a penalty brake application.
- When speed is reduced to less than 40 MPH, the high speed whistle will stop and the acknowledging horn will sound.
- Acknowledge this horn. If the cab signal continues to display Restricting, speed must immediately be reduced to restricted speed,
If restricted speed is not reached within 70 seconds after the acknowledging horn was acknowledged, a penalty brake application will occur unless the brake valve handle is in SUPPRESSION
17.5.2: Under 40 MPH
The acknowledging horn will sound if the cab signal changes from Clear to Restricting when the speed is under 40 MPH.
- Acknowledge the horn within 6 seconds to prevent a penalty brake application.,
- If the cab signal continues to display Restricting, train speed must immediately be reduced to restricted speed.
If restricted speed is not reached within 70 seconds after the acknowledging horn was acknowledged, a penalty brake application will occur unless the brake valve handle is in SUPPRESSION.
Basically if you are traveling over 40mph, reduce to 40mph and then you have 70 seconds to reduce speed to Restricting (20mph) or at least be making a sufficient brake pipe application to reach that speed after 70 seconds. It's not as efficient as a traditional 3 aspect system, but it likely won't penalize a typical train more than a couple of minutes. The situation is a bit dicier approaching diverging routes and, especially where no waysides are present, the engineer has to pretty much be prepared to advance on the slowest route unless the home signal can be otherwise made out.
Ultimately the system works, and although it dis not as advanced as the PRR version, it did save a lot of expense equipping locomotives with code following relays. The IC eventually removed it's installation on the Champlain District sometime in the late 80's or early 90's as Canadian National came on the scene. It also goes to show that the engineer doesn't need to know he's approaching an occupied block, just sufficient time to show down before entering the occupied block.