Read the textbook passage. Then, choose the correct answers. 1 What is the passage mainly about? A The history of railroad blocking systems. B How automatic blocking systems work. C A comparison of manual and automatic block systems. D Common problems experienced in automatic block systems. 2 What can you infer about blocking systems? A Single track systems have block signals facing in opposite directions. B They were impossible to make before electrified rails were developed. C They reduce the minimum safe braking distance between trains. D Lines with more than two sets of tracks use different signal systems. 3 How are single track ABSs more complicated than double track ABSs? A Their tracks need multiple track circuits running in opposite directions. B Their tracks have more switches and signals per interlocking. C They are divided into smaller blocks and use more block signals. D They must maintain distances between trains in two directions.