Could someone tell me why or where it was practical to use signal gantries? At one time I thought it had to do with space restrictions around tracks but I've seen gantries in use on double main lines where two stand alone signals should have been adequate with plenty of space. I would assume stand alone signals are lower cost to the railroad also.
Signal Gantries
- Thread starter dricketts
- Start date
justinroth
Well-known member
I believe tree growth on trackside as well as after a curve are practical applications. I have often wondered this myself.
cascaderailroad
New member
Probably they wanted a high visibility signal directly over the track, for better visibility on curve locations, where a trackside signal would be eclipsed by an obstruction.
Agreed, without more specific examples it becomes difficult to lay out possible reasons. The old New York Central 4-track main used full out bridges to span all four tracks. However where this line entered the Buffalo Terminal area only the number 1 & 2 tracks were signalled at a certain location so there is a rather unique cantilever gantry there.
More to the point, it depends on whether you're referring to recent builds or long-standing signals as well. In the days of steam signals had to be aligned to a certain side of the tracks for visbility - on the right in the US, for example. With muti-track mains there were two ways to accomplish this - "Bracket" style or gantry/bridge. This continued into the early days of diesel when LHF (Long Hood Forward) was still the name of the game. Recently though as wide-nose cabs took center stage, it became less important which side of the tracks the signal was on, as long as it was clear which signal was governing your track. Most times the status quo remains (as an old gantry or bridge) until there is a reason to spend the money replacing it. This could be a mass signal replacement (as CSX is undertaking), a derailment which took out the structure, or I think the least common issue is structural but I guess its possible.
You still see new builds use gantries/bridges for various reasons. Most often with a basic two track main would be sight lines probably, where the added height is beneficial to the engineer recognizing the signal as early as possible, and since brackets are "out of style" due to further visibility issues with the height of double stacks and other tall cars, gantries are the way to go. I can assure you they are more expensive to build so railroads are not going to go out of their way to build them unless there is a valid reason that would be dependent on each individual location.
More to the point, it depends on whether you're referring to recent builds or long-standing signals as well. In the days of steam signals had to be aligned to a certain side of the tracks for visbility - on the right in the US, for example. With muti-track mains there were two ways to accomplish this - "Bracket" style or gantry/bridge. This continued into the early days of diesel when LHF (Long Hood Forward) was still the name of the game. Recently though as wide-nose cabs took center stage, it became less important which side of the tracks the signal was on, as long as it was clear which signal was governing your track. Most times the status quo remains (as an old gantry or bridge) until there is a reason to spend the money replacing it. This could be a mass signal replacement (as CSX is undertaking), a derailment which took out the structure, or I think the least common issue is structural but I guess its possible.
You still see new builds use gantries/bridges for various reasons. Most often with a basic two track main would be sight lines probably, where the added height is beneficial to the engineer recognizing the signal as early as possible, and since brackets are "out of style" due to further visibility issues with the height of double stacks and other tall cars, gantries are the way to go. I can assure you they are more expensive to build so railroads are not going to go out of their way to build them unless there is a valid reason that would be dependent on each individual location.
There was a time that wayside signals had to visible from the rightside of the locomotive cab. Many railroads complied by using gantries and or bridges when space was tight or visibility was limited. Mountain subdivisions liked them as curves greatly diminished visibility. Always check with the railroad you're modeling as each railroad had their own way of doing things. 
cascaderailroad
New member
Why would a RR bother to erect 5 separate stand alone trackside signals, when they could simply erect 2 signal bridges ?
I would think the cost would be about the same, and underground wiring would be more complex for trackside signals.
In order to erect a right hand side pedestal signal for every track, the track spacing would have to be much wider.
A high speed mainline, sometimes requires signals directly placed overhead of a track.
And track side signals are more prone to being misread by train crews.
This is a perfect example of what I was talking about. At the time the bridges were put up signals needed to be on the right side for steam locomotives. Track spacing obviously doesn't allow for masts place between tracks as cascade said. If these signals were at one time upgraded after the days of steam it was likely possible they could use the existing wiring saving on costs but honestly the actual signals could very well be 50 or more years old anyway.
As it happens, the bridge on the far side of the road (the other set of absolutes), even if completely replaced in this day and age would probably still require a bridge or gantry, because it would be otherwise impossible to signal the middle track since a standalone signal would never fit between tracks, at which point you're building a structure overhead anyway, so why build two more masts on each side when you can put all the signals on the same structure?
My guess is signals are placed on gantries for the following reasons in order of importance to the railroad. Keep in mind there are always exception to the rules, but this is my WAG.
Increase the height of the signal for better visibility and reduce any interference from ground objects. By having the signal higher it may also reduce glare off of other equipment and the rails, a problem in wet weather. It also gives a vertical separation from the headlights of oncoming traffic.
The signals usually protect a plant or junction with crossovers and or diverging tracks.
Because the signaled area usually has two or more parallel tracks that have high traffic volumes and where space might be an issue.
Consolidation of equipment, i.e. ease of wiring and maintenance.
Reduce the potential for vandalism.
Increase the height of the signal for better visibility and reduce any interference from ground objects. By having the signal higher it may also reduce glare off of other equipment and the rails, a problem in wet weather. It also gives a vertical separation from the headlights of oncoming traffic.
The signals usually protect a plant or junction with crossovers and or diverging tracks.
Because the signaled area usually has two or more parallel tracks that have high traffic volumes and where space might be an issue.
Consolidation of equipment, i.e. ease of wiring and maintenance.
Reduce the potential for vandalism.
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Possible. Maybe it's just the perspective and field of view of the picture, but it looks like it could be a tad narrow based on the spacing of the existing tracks. That said, it isn't unheard of over time to spread the spacing apart when other tracks are removed. Not likely, but not impossible. The converging track on the right could have easily continued under the bridge, and a 4th track could have diverged off to the left coming towards the picture taker. Clearly however this possible 4th track did not continue under the next bridge.
It is also interesting to note that the two bridges pictured are of slightly different construction, perhaps due to the difference in width? (Near bridge angles are \\\\//// whereas the far bridge angles are /\/\/\/\. Also there is a serious lack of proper catwalk on the near bridge.
jkinzel, honestly I would say the space issue is really the only one that holds much weight. The increase in height to reduce interference of ground objects being the nearest second place. But if this were really a concern why wouldn't railroads have built gantries/bridges and made signals higher even on single tracks?
The idea of a vertical separation between headlights and signals is shaky at best. There is a rather obvious difference in lights that in most cases would not confuse an engineer - besides color differences, signals are focused to "show" for a very long ways without throwing "light" into an area, where as headlights are (un)focused to "light" an area as well as possible, which actually means at a distance they become less pronounced. It could be a benefit, but only in cases where you have two tracks running against each other, which likely means space between tracks.
Most signals in fact protect a junction of some kind, with the exception of automatic signals, but either way the name of the game is protection - automatics are no less important than absolutes.
Trust me, gantries are no harder to get at than waysides if you really want to vandalize something.
I'm fairly certain the added cost of building a signal bridge would have far outweighed the cost of running signal cables to individual waysides.
Not saying your thoughts are bad, it never hurts to be inquisitive. However most of the reasons basically roll back to space issues between multiple tracks, in the days when signals needed to be on a specific side of the track they are protecting.