What's the distance between signals
What's the distance signal's aspect
What's the type of signal, Absolute or Permissive
http://www.railroadsignals.us/basics/basics4.htm
There are a  number of "speed" terms associated with signals and the rules.  Some speed  terms are associated with just signals, and others are used in instructions by  the dispatcher  
 The  following terms are taken from a 
Southern Rwy  rulebook  and a 
Conrail signal  card.  There may be additional differences on other railroads.
 1) Normal Speed:  The maximum  authorized speed.
 2) Limited Speed:  A speed  not exceeding 45 miles per hour.  Some railroads, such as the New York  Central (aka: CR) specified 45MPH for passenger trains, and 40MPH for freight  trains.
 3) Medium Speed:  A speed not  exceeding 30 miles per hour.
 4) Reduced Speed:  Proceed  prepared to comply with flagging signals and stop short of train or  obstruction.
 5) Restricted Speed:  Proceed  prepared to stop short of another train, obstruction, or switch not properly  lined and look out for broken rail, but at a speed not exceeding 15 miles per  hour.  Conrail specified 20MPH outside interlocking limits, and 15MPH  inside.
 6) Slow Speed:  A speed not  exceeding 15 miles per hour.  Some railroads have pushed this up to 20MPH,  such as CSX.
 7) Yard Speed:  A speed that will permit stopping within  one-half the range of vision
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John