Signal Aspects

[Anonymous]

I need some information about signals. In Kirkwood junction, MO, UP has a signall with three heads. The top two have the normal red, yellow, and green lights. The bottom head only has two lights, and one is red. Does anyone know what the other is? I'm guessing green, but I've never seen it light. I'm making a model of it, and would like it to be as prototypical as possible. Thanks in advance.

Ian

[Anonymous]

I'd doubt that it would be green. The bottom light is for the slowest route possible (tightest turnout or trackage). It would probably be yellow, though there's a good possibility that it is lunar (white; indicates restricted speed between red and yellow). Green means 'go ahead at track speed,' which is rare for diverging.

Then again, I've never seen the signal in question.

[Anonymous]

I also suspect the second aspect which might be displayed by the bottom signal head here would be either yellow or lunar, but for different reasons.

Under the block and interlocking signal rules traditionally used by UP and several other U.S. roads, a red-over-red-over-yellow or a red-over-red-over-lunar aspect indicates "restricting." "Restricting" means "proceed prepared to stop short of train, engine, obstruction, or switch not properly lined, looking out for broken rail or anything that may require the speed of a train or engine to be reduced, but not exceeding 20 MPH." A restricting indication might apply to either the normal or the diverging route at a junction.

The alternate indication on the lower head probably wouldn't be green, but not for the reason previously claimed. Since this signal can display a green aspect on the middle signal head, it can indicate "divering clear" with a red-over-green-over-red aspect. It wouldn't make much sense to have a green as the lower aspect, since red-over-red-over-green would just be another way of indicating "diverging clear" and wouldn't permit the display of "restricting".

"Diverging clear" in advance of a junction or crossover is a very commonly-seen signal indication--it is hardly a rarity. "Diverging clear" means "proceed on diverging route at prescribed speed." The employee timetable or special instructions will set the speed for the normal and diverging routes at the particular location.

[dehusman]

It would be Red and Lunar. Since clear and stop have to be two of the indications, the only way you can get those is with a red on the bottom, so one color has to be red. Every other indication that would be given there (with one exception) can be given with the three colors (R-Y-G) on the top two heads. The only indication that would require a different color (lunar) and could be on the bottom head is restricting. The only time you will see a lunar there is if something is lined off the main onto the Kirkwood Industrial Lead.

Dave H.