Western Railroad Discussion > Signal question

used to see signals that would changed from green to red immediately when a locomotive or car passed the signal.  This does not happen now.  Several engines and or cars have to pass the signal before it reacts.   Why?  From looking at the track at the signal there is still an insulated rail joint.

         The signal turns red as it is supposed to, why be concerned about another second or two for it to happen. Concern is when it never happens.

No concern, just wondered why the change.

Used to be track relays that would respond faster than the electronic logic now used.  

tomstp Wrote:
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> used to see signals that would changed from green
> to red immediately when a locomotive or car passed
> the signal.  This does not happen now.  Several
> engines and or cars have to pass the signal before
> it reacts.   Why?  From looking at the track at
> the signal there is still an insulated rail joint.

tomstp:
 
You made a very good, astute observation!
 
It would be helpful to know where you saw the signals change quickly, and where they now change in delay fashion, and the railroad(s) involved.
 
If my area of Southern California, the most prominent railroads are BNSF (old Santa Fe) and UP (now with SP), plus a few commuter outfits.  Sixty years ago, UP’s signals delayed whereas AT&SF’s and SP’s were immediate.  I remember riding UP’s City trains and would see five E-units and several passenger cars pass a green signal at 60 M.P.H., and just before the dome car was reached, the green signal would finally change to red!
 
In my opinion, UP was and is more progressive in some ways.  Having delays in signal changes, the railroad avoids sudden false reds.  The signal system makes sure a train did in fact pass the signal before it changes to red.
 
Hope the above answers your question satisfactorily and broadens your understand of a fascinating industry.

As SP4360 alluded to, electronic track circuits (e.g., Electrocode) take several seconds.to change state, so typically several engines/cars pass the signal before it drops to red.  Electronic track circuits are generally used at intermediate signals.  DC/relay track circuits are still generally used within control points, so the signals at control points react almost immediately.

AB and Bob, thanks for your answers.  You covered by questions.

Just for clarification, on UP, whether a signal was an intermediate or an absolute, they acted the same 60 years ago, and both types changed as the dome car approach the signal.
 
Currently, it is unclear what the electronics are, but various subdivisions have various systems of varying age, thus, only a field check would show how a clear signal would respond when a train knocked it down.