Signal question

[charlesriverbranch]

I was at Anderson/Woburn the other day waiting for a train to Maine. A Lowell-bound train had just departed, and the signals showed red over red. After a few minutes they changed to yellow over red, but a few minutes after that they changed to yellow over green. Then, shortly before my train arrived, they changed to green over red.

I know that green over red means clear, and yellow over red means that the next block is clear but the one after that is occupied; but what does yellow over green mean?

[nomis]

The signal progression you observed was Stop, to Approach, to Approach Medium, to Clear.

yellow over green is: Approach Medium - Proceed approaching the next signal at Medium Speed. Typically when a track is occupied two blocks ahead. Check out this site for more info.

[train2]

On many railroads, most in fact, an Approach Medium, yellow over green, is a signal used to indicate a diverging move at the next distant interlocking. However some railroads use it as a quasi-Advance Approach, meaning as others have stated a train is 2 blocks ahead. Its all how the rules for the particular railroad are implemented. Since your progression upgraded to clear it was not for a diverging move, but just an advance warning of a occupied block 2 ahead.

[BostonUrbEx]

To further add to the information, the MBTA uses Approach Medium instead of Advanced Approach because it allows trains to travel slightly faster. You can pass an Approach Medium at track speed, but you must immediately begin reduction to Limited Speed (45 mph) when you pass an Advanced Approach. Advanced Approach signals also make signal systems more expensive because they require the top light on the mast have a flashing mechanism (flashing yellow, over red, over red), so the T saves some money on this.

[Trinnau]

To add a little more, once you got on the Downeaster and left Anderson/Woburn the next signal you pass would have been Approach Medium leading to a Medium Clear for the diverging move to the Wildcat. So you experienced both uses of the signal without realizing it.

[AmtrakLocomotiveEngineer]

The Approach Medium could take you to a Medium Approach as well.

[sonicdoommario]

To further add to the information, the MBTA uses Approach Medium instead of Advanced Approach because it allows trains to travel slightly faster. You can pass an Approach Medium at track speed, but you must immediately begin reduction to Limited Speed (45 mph) when you pass an Advanced Approach. Advanced Approach signals also make signal systems more expensive because they require the top light on the mast have a flashing mechanism (flashing yellow, over red, over red), so the T saves some money on this.

There's a set of signals in Attleboro over tracks 2 and 4 just past the Hebronville interlocking. The signal over track 4 can be a flashing green over a red (leading to a yellow over a flashing green at the station itself), although I think trains can still only pass that flashing green over red at 45mph. Does the flashing indicate that the next signal will be different?

[WhartonAndNorthern]

Ok so Attleboro's on the NEC so NORAC rules apply. Flashing green over red is "Cab Speed" (Rule 281a). That means that the engineer determines the track speed by looking at the display on his cab signal readout. If the readout displays 45 mph, then that's the speed. Yellow over flashing green is "Approach Limited" (Rule 281b): expect the next signal to be no more favorable than "Limited Speed" (45 mph). Usually a Limited Speed signal (Limited Clear as NORAC doesn't use Limited Approach) indicates that the train will switch tracks over a limited speed turnout. A railroad could conceivably use "Approach Limited" as a warning before an "Approach Medium."

[MBTA F40PH-2C 1050]

As long as we are not going to hold in Attleboro for an Amtrak to overtake on Track 2, the normal signal progression for our trains is Cab Speed at HEBRONVILLE INT for the move from Track 2 to Track 4. The Automatics at Dodgeville (195.1W/195.2E) will display Cab Speed on track 4 (45 mph).... Approach Limited at BORO, Limited Clear at HOLDEN for the move back to Track 2, and onto Mansfield with high greens

[WhartonAndNorthern]

While the wayside's displaying "Cab Speed," since the actual speed is set to 45mph, would the older ACSES and PRR-style cab signal displays show the "approach medium" symbology?

[CRail]

A couple points... On a cab speed indication, the engineer uses the speed displayed in the cab to determine signal speed. Track speed is permanent. I know it sounds like a nitpick but it’s an important distinction.

Approach Medium does not need advance warning as you can pass it at track speed. The indicated speed does not need to be adhered to until the next signal though reduction in speed must begin as soon as the first signal is passed.

[jonnhrr]

Does the flashing indicate that the next signal will be different?

The railroaders can correct me, but I believe that a flashing head "upgrades" a signal over the corresponding indication if the light was steady e.g. limited speed 45 mph vs. medium speed 30 mph.

Jon

[ExCon90]

Correct--that way, if the flasher should fail, the resulting (solid) aspect is less favorable than the intended one. I only know of one exception: on the Chicago Metra ex-SOO now-CN line to Antioch they introduced a Diverging Clear Approach Diverging (which is nonsensical; either it's Diverging Clear or it's Diverging Approach Diverging), displaying Red over Flashing Green, while Diverging Clear is Red over solid Green. It's possible to design the circuitry to knock the signal down to Red over Yellow if the flasher on the bottom unit fails--I hope they did that.

[train2]

Actually cab speed goes against the flashing upgrades logic. It would be fo
some speed less than track speed.

[Head-end View]

Another interesting use of "approach-medium" is to enforce a lower speed limit thru a given area. Long Island Railroad recently started using it to enforce an already existing 40mph speed limit for eastbound Main-Line trains thru the Hicksville Station complex. That signal used to display clear but with railroads now using signals for this purpose now the most favorable indication for Main-Line trains is the approach-medium.