What does the term "railed" mean in signal processing? I'm having trouble finding a definition of "railed" that relates to signal processing.

Am I correct in my guess that this term is in fact from this field?

My signal data comes from an EEG device. The lightly documented open source software I'm using doesn't define it, but it shows that term when there is no signal data being displayed.

Is that all it means (no data)? Or does it mean something like the signal being read is too great to be displayed or correctly measured?
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.