Hi all, I’m wondering what is the main porpuse between the Double Head and the Offset Double Head Search Lights. I model the modern CN and I see they use both types. Can anybody tell me why the “Offset”, please?
The vertical lights indicate a home signal - absolute in that without a M.P. numberboard the signal is a “stop-and-stay” indication at red-over-red.
The offset heads for a block signal - permissive in that a red-over-red indication allows proceed after stopping. The second head provides an advance indication for a following signal in the next block or control point.
By setting the two heads offset it gives the crews notice that it isn’t a home signal.
Hope this helps.
D.M. Mitzel, Oxford, Mich. USA
Not really.
Look at the signals displayed. Both show a “clear” indication. Green over red.
So if the second head shows an “advance indication” of the next signal that would mean the next signal was red? Don’t think so.
You have to take the whole signal, all the lights into consideration. You can’t separate the lights and assign each light to a different switch or block or whatever.
Dave H.
Dave,
I wasn’t really explaining the aspects/indications displayed in the photos. The OP’s question had to do with multi-head searchlight signals and the difference between vertical and offset placement of the heads, not necessarily the indications shown in the photos (which are cut-and-paste out of Tomar’s catalog). I was merely trying to explain the difference between an absolute signal (first photo) and a block signal (second shot). I used the red-over-red example to highlight the operational difference between the two - that a crew must stop and stay (cannot proceed past red without operator/dispatcher permission) at an absolute signal, while a train can proceed past a stop indication at a block signal at restricted speed, expecting a train or obstruction in the block.
Now, regarding multiple indications that can be displayed with dual-head absolute and block signals, there are resources on the web where the OP can further research this depending upon their chosen rules/territory/prototype (UCOR, NORAC, etc.). However, I felt that this was beyond the scope of the OP’s original question and could not be accurately answered without knowing the prototype in question.
Regards,
D.M. Mitzel, Oxford, Mich. USA
The upper example (two or three lights one right abover the other) is also often used at ‘interlockings’ - places where tracks cross or diverge.
If you were coming along a mainline and a branchline was going to split off of it to the left while the mainline continued straight, you could see one of the following indications:
Green-over-Red = You’re continuing straight on the mainline
Red-over-Green = You’re taking the diverging route (the branchline)
Red-over-Red = Stop - absolute stop.
You wouldn’t see Green-over-Green - the switch can’t be set for both tracks at the same time!
In a sense, the upper light indicates the mainline, the lower, the diverging line. It’s kinda like at a road intersection where you have red-yellow-green stop-and-go lights for the main road, but also a second head for a left turn lane.
Unlike road stop-and-go lights where red means stop, on many railroad lines “red” signals are permissive, you don’t have to stop just slow way down. That’s handy say if a signal is on a grade, it allows a train to keep moving up the grade - if it stops, it might have trouble getting started again.
The red-over-red indication is always absolute - you must stop. That’s why you’ll sometimes see this used at say a place where two railroads cross at grade. A “dummy” head will be used which only shows red, so you can display red-over-red (stop) or green-over-red (go thru). It just reinforces that the train must stop short of the signal.
Actually you could because its an aspect in some signal locations.
Once again, you can’t separate the two lights. For example a valid indication in many signal systems is yellow over yellow. If your assumption that the top signal applies to the main and the bottom signal applies to the diverging route, the that signal would mean you are lined on both routes. (That signal is advance approach which slows down a train in preparation for an approach signal).
You also have to be very careful about comparing railroad signals to traffic signals because they work very differently and mean very different things.
True with a whole bunch of qualifications. Its a stop and proceed signal only if it has a number plate on it, has a “P” (permissive) or “G” (grade) plate on it. You can only pass it without stopping if you were already moving at restricted speed or it has a P or G plate on it. A single red signal with no number plate or with an “A” (absolute) plate in CTC or at an interlocking is normally a stop signal (can’t pass without additional authority).
A red over red with no number plate or with an A plate that
Thank you very much DM, very much appreciated. Exactly what I needed to know. I was wondering because those Tomar seach lights are the ones that I will be using on my layout and I needed to know the difference and if it was worth purchasing the “offset” lights or not.
Thanks,
John