What does this mean?

[jeffhergert]

On my railroad the Yellow over Yellow, or as some put it, "double bubble" is Approach Diverging. Proceed prepared to take diverging route at next signal, not exceeding the prescribed speed of the switch.
Two yellows over a red is also Approach Diverging. Red over two yellows is Diverging Approach Diverging. Proceed on diverging route not exceeding prescribed speed of switch prepared to take diverging route at next signal not exceeding prescribed speed of switch.

[wabash1]

That signal is a advance approach . ( on the road i work) meaning the distance between the next signal and the stop signal is to short to allow a train traveling at track speed to stop safley. the true meaning of this signal is procede prepared to stop at 2nd signal train or engine exceeding medium speed must at once reduce to that speed.

[kenneo]

In the US, a double yellow would mean proceed prepared to stop short of the next signal. The next signal will be assumed to be red at that point. In short, the top light would be a block signal.

The lower light would be a distant signal for the next signal - the one presumed to be red for the block. Yellow, in this case, would mean that the lower light on the next signal would also be yellow (distant signals are "repeater" signals) and signify that the switch is lined for the diverging route and the dirvirging route is clear to the next signal, and the train is to proceed prepared to stop at the next signal, expected to be displaying red, at turnout speed.

Where a divirging route occurrs, other rules can (and usually are) invoked, such as proceeding at a speed not to exceed restricted speed, and such as that.

If the divirging route is into a siding, the only time a green should be displayed on the lower light is if the train is lined out the far end, but most railroads have dropped this provision for safety reasons. It does not normally speed up operations and can get a crew into a side collision situation if the signal is dropped infront of the train using the siding.

[JimBaker]

The Santa Fe (BNSF that is) uses a "Double Yellow" or "Yellow-over-Yellow" indication to indicate caution approaching a junction.The junction switch being aligned for the Diverging Route ahead.
This is a general statement not an actual quote from any rule book, and is off my head.
Usually a "Red-over-Green" (or Yellow) is used for "Diverging Clear or Approach". indication.

[Anonymous]

If it is only a two aspect signal in Canada it would mean Clear to Slow meaning you can continue at track speed until you get to the next signal that will be slow to something. When you get to the next signal you cannot be traveling more than 15MPH past that signal...

That's here in Canada (according to CROR), it might be different depending on where you live.

[Anonymous]

Today on my way home from work I noticed the signal had 2 ambers Instead of 1 green and 1 red. What does this mean?