While in ABS-TWC territory, what must you be prepared to do when approaching a next signal displaying a solid yellow aspect?

Answer: C — Proceed at restricted speed at that block signal

In ABS-TWC (Automatic Block Signal - Train Control) territory, a solid yellow aspect indicates to the train crew that the next signal is displaying a stop signal, which requires caution in the approach. The correct action upon encountering this aspect is to proceed at restricted speed. This regulation is designed to ensure safety by allowing the train time to respond to the upcoming signal while maintaining control. Restricted speed means that the train must be able to stop within one-half of the distance to the next signal. By doing this, the train crew can safely react to any potential hazards or situations that might arise, such as a signal displaying a stop aspect or any obstructions on the track. Maintaining current speed or proceeding at maximum speed would not provide the necessary safety margin to respond to a stop signal ahead. Stopping immediately at the next signal may not be appropriate, as it doesn't consider the need to approach the signal without exceeding the safe operational limits. Thus, the requirement to proceed at restricted speed is fundamentally about prioritizing caution and safety on the tracks.