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In general a flashing aspect lets you display a signal that would require one more head than the signal it's on ... for example flashing yellow can replace yellow over yellow, or yellow over yellow over red (etc) ... so you can do "advance approach" on a single signal head.... you go from green, to flashing yellow, to yellow, to red .... and some systems modify it further by giving you flashing yellow, then yellow, then flashing red, then red: trains can pass the final flashing red at restricted speed. So the flashing light is often used to modify the signal one step in the "less restricting" direction.

Start wtih this:

http://broadway.pennsyrr.com/Rail/Signal/aspects_us_cl.html

So ... what does a flashing green give you? Most systems list this as "Cab Speed" .... but the above formula works. Think of it as Green over Green over red (over red...etc) where now you're clear to do whatever your cab signal tells you is allowed.... a speed that might be higher than ordinarily allowed at an interlocking or in block signal territory, where generally the signal only tells you what the next two blocks are like. Most of the time, block signal territory (where there is no cab signal equipment) is limited to 79MPH so you'll really only see the flashing green/ CAB SPEED indication where speeds over 80 are allowed. Notice the explanation in the NORAC signals:

http://broadway.pennsyrr.com/Rail/Signal/aspects_us_norac.html

So ... clear as mud?


Matthew (OV)
 

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Railroad signals actually came first ... the motor vehicle traffic kind were a simplified version. Signals generally tell you one of two things: 1.) The status of a block ahead of you (Occupied, as in, there's a train there, or Clear, as in, there's no train there) and 2.) Speed control (as in, "Slow down to Medium Speed as you pass this signal, and when you get to the next signal, be going slow enough that you can stop if it says to.")

Imagine a traffic light that told you "You can pass this signal at 45 mph, but be prepared to hang a left at the next light, and be going no faster than 10 mph when you get there!" You'd have a lot more than "Green, Yellow, Red" to remember ... and it might vary from state to state, or city to city.

The other thing that can be confusing about railroad signals is that red doesn't ALWAYS mean stop. Sometimes it's just a placeholder; sometimes it means "Stop and Proceed," and sometimes it means "Stop.. and stay stopped!" So, a two story tall "Green over Red over Red" is sometimes the LEAST restrictive signal in a system (and means "Clear") while a tiny dwarf signal with a red lamp might require a train to stop and wait indefinately for a better signal to be displayed!

The broadway limited pages are really good primer if you want to understand the basics of how most American signalling (and some from other countries) works, and how it came to be. The whole thing starts here:http://broadway.pennsyrr.com/Rail/Signal/index.html

Matthew (OV)
 
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