If you want to understand US railroad signaling, study this:
Al Krug’s Signals Page (archive)
There is a logic to the system, and it’s used in different ways by different railroads, but it boils down to the same stuff. It’s only confusing until the “ah-ha!” moment where you get it.
Larry (tree) has a great suggestion regarding the Deshler web cam. It shows those great B&O CPL signals in use on the PTZ camera. What great relics those are.
I have always been intrigued and studied signals. About 50 years ago I purchased a Kalmbach book “All About Signals” which was a reprint of a couple of Trains articles. Still have it…somewhere.
ed
While “All About Signals” is rather dated (the TRAINS articles were from 1955). it is still useful for giving the basics of both over the road operation and interlockings.
Some signals are speed, some are routing. Even the speed signals can give an indication regarding routing if you’re familiar with the track layout.
At risk of repeating myself (I didn’t go back to look at what’s been written previously), standard signals assume three heads. Top head is track speed (whatever it happens to be). Middle head is medium speed, bottom head is slow speed. If there are less than three heads, assume them to be missing from the bottom up, and assume them to be showing a stop.
So, a “high green” is a signal to proceed at track speed. A middle green indicates that a train must reduce to medium speed (defined in the timetable), and a bottom green means proceed at slow speed. As mentioned, it varies by railroad, but it’s kind of a rule of thumb. Yellow in any of those positions indicates that the next signal may be red (stop) and to proceed accordingly.
There are myriad variations - flashing, multiple heads in different colors, etc. It’s best to find a signal chart for the railroad in question and look up the governing rule for each signal indication.
B&O CPLs are a world unto themselves, but actually really easy to understand once you get the hang of them.
As mentioned, signal indications can vary by railroad. This was the root cause of a rear-end collision involving Amtrak in the Chicago area several years ago. The engineer didn’t account for having changed host rail
Never been a fan of using a red-over-yellow signal for a restricting indication. I live on a former N&W line, so red-over-yellow is a Restricting but red-over-yellow-over-red is a Diverging Approach. That is not the case for NS’ former Southern territory. This subtle inconsistency is dangerous in my opinion, especially if crews operate over routes with multiple sets of signal rules. Lunar would be a much better standard color to use across the industry for restricting aspects. I know - not going to happen, it would cost too much to standardize. Safety first! Right…
About 15 years ago the local NS dispatcher caught a UP crew operating on joint NS/UP trackage miscalling a yellow-over-yellow-over red signal an Approach Diverging rather than an Advance Approach. Could have caused an accident because an Approach Diverging tells a train to slow down and prepare to proceed thru a diverging route. An Advance Approach means stop at the second signal. In this particular instance, due to short signal blocks, the second signal was only 5,800 feet away. I was impressed by the DS’ keen ear and instant understanding of the ramifications. Of course, he was an “old head” with 30+ years on the railroad. I have no doubt that he had studied all of the signalling charts on the districts that he dispatched and knew all of the signal aspects, names, and indications that each signal on his territory was designed to display. By signalling charts, I mean diagrams of the inter-relationships of signal aspects between signal locations along each route, not the basic signal rule charts.
Not all carriers have rules in place requiring crews to call out on the Road Radio Channel the signal indications and location of what they are seeing.
Additionally