block signal

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A fixed signal at the entrance to a railroad block, indicating whether or not trains may enter.

from The Century Dictionary.

• noun An automatic signal placed at one end of a section of a railroad to block the section by warning trains coming into it of the presence in it of another train.

from the GNU version of the Collaborative International Dictionary of English.

• (Railroads) One of the danger signals or safety signals which guide the movement of trains in a block system. The signal is often so coupled with a switch that act of opening or closing the switch operates the signal also.