Final answer:
The signals that allow a train to close in on another train ahead at congested stations are known as block signals. These signals are part of a critical safety system that prevents collisions by maintaining safe following distances between trains.
Step-by-step explanation:
The signals used at congested stations to permit a train to close in on the train ahead are called block signals. Block signals are part of a safety system designed to control train traffic and prevent collisions by dividing the railway into sections or 'blocks'.
Each block contains signals that indicate whether the section of track is occupied or free. These signals ensure that a safe distance is maintained between trains, allowing for smooth and safe train operations even when stations are busy. In the context of train stations, these signals are crucial, as they manage the flow of numerous trains entering and leaving the station, and thereby play a significant role in maintaining the punctuality and reliability of the rail service.