What does the term "railed" mean in signal processing?
A railed signal, or a railing signal, seems to indicate a flatline. On BIOPAC, Railing signal (flatline) says:


  When the amplified signal for any given channel exceeds the range -10
  to +10 volts, the signal will rail. You will see a straight line at
  -10 or + 10 volts (more likely the reading will be close 9.99 volts). The MP system is designed to work only in the range -10 to +10 volts.
  The signal could rail for several reasons (which are not exclusive)...


From Amplifiers: What do rail-to-rail and single supply mean?


  With respect to analog signals, a “rail” is a boundary that a signal
  has to work within.