--David
QUOTE: Originally posted by BR60103 Block distances were usually dependant on braking distance: a train should be able to stop from track speed between the yellow signal and the red one. Where signals were closer together for denser traffic, they would have extra signal indications like double yellow to allow the engineer to slow the train down. The Toronto subway has blocks about a train length (500 feet) and keeps 2 red signals and a yellow behind a train.