Doug:
Cab signals are no longer in service between Aurora and Savanna. Since they were only on passenger locomotives, there were no locomotives with cab signals operating on this line after May 1, 1971. The removal of the cab signal field equipment may have occurred before that date, I'm not sure.
The January 1971 issue of Railway Signaling and Communications contained a news article indicating that BN had asked the FRA for permission to discontinue its automatic cab signal system on approximately 105 miles of mainline between Aurora and Savanna, Illinois, and also on all through (not commuter) passenger trains between Chicago and Aurora. The article went on to say that BN had 65 locomotives equipped with cab signals, that the cost of motive power was such that these locomotives must be utilized system-wide, and that these locomotives could be more efficiently operated over the entire system rather than being limited to Aurora-Savanna and Chicago service. The article also stated that maximum speed had already been reduced from 90 mph to 79 mph on the territory west of Aurora. I did not find any subsequent information on when the FRA approved this request.
Burlington Northern Special Instructions #2, dated October 25, 1970 appears to verify the speed reduction, as it lists the maximum speed at 79 mph. The speed reduction could have occurred on that date, or by Superintendent's Bulletin prior to that date. Others on this list who may have been in train service at the time may have knowledge as to exact dates for the speed reduction and removal of the cab signal system.
While the speed reduction was officially made by Special Instruction, it is my opinion (and I'm not trying to point a finger at any operating personnel) that, practically speaking, it was not adhered to. Between the fall of 1970 until Amtrak took over and passenger service was removed on the former CB&Q between Aurora and St. Croix Tower, I don't think the schedule of any first class trains was adjusted to reflect the lower maximum speed. And I personally rode two trains over the territory during this time (the eastbound Empire Builder - North Coast Limited and the westbound Morning Zephyr) and timed both trains at various locations west of Aurora at, or slightly above, 90 mph between concrete mileposts.
Glen Haug