1.   But as we go back to much earlier periods, the signal systems are complex in different ways again.

2.   Meanwhile the borderline case is important in understanding the complexity of primary signal systems.

3.   Amtrak is also replacing a signal system so antiquated that bridge tenders operated according to train timetables.

4.   Based on the Toronto Transit findings, the Transit Authority hired another consulting firm, Parsons Brinkerhoff, to study the entire signal system.

5.   After the accident, the Transit Authority hired a Toronto engineering company to study the signal system.

6.   But overhead wires still tangle, signal systems malfunction, switches freeze.

7.   Dallas-based Texas Instruments sold the software unit as part of its plan to focus on digital signal processing systems.

8.   Engineers often rely on signal systems to detect a train ahead on the tracks.

9.   If the signal system was working properly, the light just behind the stopped train was red.

10.   In addition, last October the authority hired Parsons Brinkerhoff, an engineering and consulting firm, to review the signal system and identify potential hazards.