This page will attempt to document the Milwaukee Road's Controlled Automatic Block (CAB) signal system that was in place in Terre Haute, IN. CAB rules were in effect between the south end of Van Yard and Belt Junction. This system was controlled by the operator at Spring Hill tower. The CAB system lasted until the late 1990s.
I have little information about the history of the CAB system due to the fact that I have just begun researching it. If you have any additional information please email me [hallbf] *at* [gmail.com].
I don't know yet when this system was first installed. The earliest date on the plans for the south end of Van Yard is 1-13-1947. The earliest date on the plans for the intermediate signals (signals 178-2 and 178-3 at MP 178.19) north of today's Deming Park is 1-4-1947. This system was removed by the Candian Pacific sometime in the lates 1990s.
In this stretch of the MILW and perhaps elsewhere as well, northbound signals were generally labeled R and southbound signals generally labeled L. R being Right and L being Left. This coincides with the signal drawings as north was on the right side of the page.
The north and south ends of College Ave. Siding had a dwarf signal governing movmements out of the siding. Signal 5B was on the north end and Signal 5A was on the south end. The CP plans I have dated 6-12-1998 indicate Signal 5B was a color light. By this time the south switch to the siding had been removed. Since I have not seen plans for the south end I assume the operating was similar to the north end. In order for a train to leave the siding a few things had to happen. First the dispatcher had to issue a request to the signal system. This was accomplished by moving lever 5 on the control panel from N to R. This applied battery to the pole line wire 5B-LS from Springhill to College Ave.
Next a trainmen would have to push a button located on the relay house near the switch. Pushing this button opened the battery to the NWP wire which went out to the circuit controller on the switch. This caused the NWPR (Normal Switch Repeater Relay) to drop. Provided no trains were lined up either northbound or southbound into the CAB territory or a train was in the block the dropping on the NWPR relay would allow the NAR and SAR relays to pick up. With NAR and SAR up, the dispatcher request and the pushbutton pressed this would apply battery to the 5B-LSR relay. The 5B-LSR had a stick circuit around the pushbutton contact so it would remain energized when the pushbutton was released.
Now with 5B-LSR picked up the dwarf signal would change from Red to Red over S if the switch at the siding wasn't lined reverse. When the siding switch was lined reverse the signal would change to Yellow.