crossing signals.......

[Mookie]

I think I have this straight - crossing gates are activated by an electrical current that is broken by the engine. Correct?

So why do some engines activate the gates about 200-250 yards down the track and others activate them when they are about 100 yards away. And why would the back of the train sometimes keep the gates down at least 200 yards after the train is past and other trains deactivate the gates just shortly after they pass.

Is there some new device they are trying out to activate crossing gates, other than circuits?

Mookie

[dehusman]

Crossings are set to activate so many seconds before the train occupies the crossing. Crossings on slower track will activate when the train is closer. Some crossings have speed sensor circuits that can tell the speed of the approaching train and so adjust when the signals come on based on the train's speed. When the gates go up may depend on how many tracks, or how it is set, or the gate operates.

Lots of different factors.

Dave H.

[Hugh Jampton]

It depends on how the system is all wired up. It's simple enough to have a track circuit that activates the gates a certain distance either side of the crossing, so you have one track circuit that covers the whole crossing. Trains coming along hit the circuit, the relay drops and the gates activate (this is called the striking distance). As there's only one circuit the gates will remain down until the train clears the circuit at the other side of the crossing. If you want the gates to raise as soon as the train has passed the crossing then you need to install insulated track joints immediately beyond the crossing (in both directions). You can look for them next time you're out. Now you have 3 track circuits which means that you have to have 3 power supplies, 3 relays (which have to have more contacts on them), and a load of insulated block joints, and,, the wiring of it all is much more complicated. These bits are all subject to different types of failures which can make the crossing more unreliable. So if it's an unbusy road then it's cheaper for the railroad to keep it simple and just use 1 circuit and let the motorist sit for an extra 30 seconds or so.
In the UK some crossings are activated by what's called a treadle, which is a mechanical switch operated by the wheel flange because if you use track circuits for crossing activation they have to be plumbed into the signalling system as well, which further increases complexity.
Research is ongoing to try a number of different methods including motion detectors, vibration sensors, cameras connected to computers, radio etc. The problem with these methods is making them fail safe

[Anonymous]

Let's see If I can properly draw a Diagram-


N===1====================2|------||-----|==3=================4====S

Legend:
=== RR tracks
|------||------| Road
1,2,3,4 Insulated Track Joints
N,S = North and South

So, in accordance with the now outdated system, still used on branch lines and other relativelty quiet Railroads.

A train is heading from North to south-
As it croses link One, it turns ON the Railway lights-
As it croses link 2- it lights a postition marker in the internal working of the logic box for the RR crossings (big silver boxes that contain the workings of the lights etc..) So now the lights know the Front of the train is just heading over the crossing
As it croses link 3- The end of the train will shut of the RR lights as the last car pases as the current is disrupted-
As it croses link 4- this is the msot important link of all- it creates a closed circuit- 1,2,3,4 have succesfully been passed, and the system is ready for another train.

Problems with this system-
1,2,3,4 (north to south) or 4,3,2,1 (south to north) That is the exact order in which the RR lights have to have the train pass over them- In case one breaks for example- and the Train pases over them like so..

1,2,3,4

Then the RR lights will activate fail-safe, that is when the blasted things chime until the reset button is pressed.

Should at any point inbetween link one and four, the track current come undone, the wire pops out, Fail safe again- as it has lost track current.

A complete circuit msut be ataiuned- on the odd ocasion, Link 4 may fail to work, so a train will activate on link 1, and shut off on link 4- Now usually the system gives the trin 60 minutes to run link 4, and complete a full circuit- if this link breaks and it times out, failsafe will be activated again-

thats a basic description of a single crossing-

add a siding or another track or even another road like so

N==1========2==|---||---|==3=========4==|------||-------|==5==========6===S

then things complicate even more.

[CShaveRR]

There are chickens out there who know more about this than I do.

However, the circuitry in our area is such that the speed of the trains determines how early the lights and gates are activated. The commuter trains coming westbound into Lombard drop the gates at Elizabeth Street from nearly a half-mile away...then the train stops at the station. There must be a timing mechanism in there, because the gates go back up while the train is stopped, and drop again shortly after the train starts moving. Eastbound, it's the same story--fast-moving commuter trains are barely in sight before the gates activate, but slow-moving freights can creep up startlingly close before anything happens.

[rrnut282]

Newer installations use a set frequency across the rails (AC instead of DC) and use dopplar shift to detect train speed to determine the exact time to drop the crossing gates. (The same principle that makes the air horns sound different as a train approaches and departs. Sound waves in the air and voltage waves in a wire) The controller compares the results from several points around the crossing. This is superior also because you don't need as many insulated joints at multiple crossings. Each crossing uses a different frequency and can work independantly from the others, if desired.

[MP57313]

CShaveRR,

Yes there is at least one knowledgable chicken out there

The transit lines in LA (Blue Line and Gold Line, where no freight trains operate) have some sort of timing mechanism built in at the crossings next to stations. According to one of the local experts, these trains could not operate in "express mode", end-to-end with no stops, because the gates next to stations are not set up to drop on approach. The trains need to stop at each station for a set time and then the gates will be lowered. Crossings that are not next to stations (Spring St. in Long Beach) seem to work just like those of freight lines, where the gates do drop on approach.

[Junctionfan]

How come when it rains, I have seen gates go down only to go back up a seconds later and there isn't a train insight?

[Mookie]

QUOTE: Originally posted by Junctionfan

How come when it rains, I have seen gates go down only to go back up a seconds later and there isn't a train insight?

And when it is extremely hot or cold, they do the same thing. It is just the imperfect world we live in. Weather has its effect on lots of things - railroad signals included.

Mookie

[Mookie]

They just completed some work around this particular crossing. It is used quite a bit by the general public and all the trains through here are fairly slow - coming and going out of the yard.

However, the crossing gates go down so long before the train gets there, it encourages people to go around the gates rather than wait the at least 30 seconds for the train to get there. And they are doing it more all the time. Carl and Nora will know what I am talking about - the train triggers the crossing clear down at the bridge - about 250 yards from the xing. Moving so slowly, people have a clear view and just keep crossing. What they don't have a clear view of, is the one coming around the building from the south, while they are watching the one coming from the west!

That's why I wondered if they replaced the old sensor system with something new and to my way of thinking, not nearly as good!

Mookie

[rrnut282]

Are there a lot of cross-overs or track swithes nearby? Switches can play havoc since it is hard to completely insulate them especially when the points are moving. I have watched gates "pump" while a nearby power switch was being thrown. Once things were "lined and locked" the gates settled down. (make that up[;)] ) The more tracks and the more combinations of routes through a grade crossing, the harder it is to eliminate the "sneak circuits" or momentary shorts as wheels cross the gap in insulated joints, etc...

As Scotty said on Star Trek, "the more complicated the plumbin, tha easier it tis to stop up the works."

[Hugh Jampton]

The length of the crossing circuit is determined by the line speed (the speed of the fastest train). In ye olde days there may have been passenger trains on this bit of track (they tend to go faster). Since the passenger trains stopped running no one has changed the length of the circuit although this wouldn't necessarily happen if the line speed is still the same. Hence the inordinate striking time which leads to people not waiting.

[Mookie]

There are six tracks, so yes - a lot of switches. It has only been just before they started construction on this area that this has been a problem. About the time they removed my beloved "dwarfs"!

Before that, the gates seemed to go down just about the right amount of time. It is a puzzler!

Mook

[CShaveRR]

Are they done with the work there yet, Sis?

Maybe the current situation is a "work in progress".

BC

[Mookie]

QUOTE: Originally posted by CShaveRR

Are they done with the work there yet, Sis?

Maybe the current situation is a "work in progress".

BC

The crossing is open, the new concrete ties are laid - and except for maybe a couple more ties to put in, looks pretty complete. Those signals are so screwed up - they haven't been right since you visited them!

So what did you do?

Waiting to see what they finally end up doing - since the ones we sit closest to, don't even make any noise - just flash. The bells don't ring at all! Don't want to heckle the powers that be until I am sure they aren't going to do something with them. Then, will call and tell them and ask for my dwarfs back, too!

SJ

[Hugh Jampton]

If they were fine recently but are not now, then perhaps there is still a slow order on the line. Sometimes it takes a while for them to get lifted becasue the track beds to bed into the ballast a bit and may be waiting for a tamping.

[tree68]

I've seen many times when the gates drop for an approaching train, only go finally go back up when the train stops short of the crossing. On the other hand, when the train starts rolling again toward the crossing, it only has to move a few feet before the gates drop again

.

[Mookie]

QUOTE: Originally posted by Hugh Jampton

If they were fine recently but are not now, then perhaps there is still a slow order on the line. Sometimes it takes a while for them to get lifted becasue the track beds to bed into the ballast a bit and may be waiting for a tamping.

Hugh - they were "normal" before the work started in June. They are now just crazy. They go up and down for no reason (and it hasn't been that hot!) and they block traffic way to early for a train that is just starting to move. 200-250 feet is way to early and encourages bad drivers. So hope you are right and they will correct the problem pretty soon. Otherwise, I am going to start carrying big blotters in Millie... Ugh......

Mook

[rrnut282]

If the trains are going fast enough when they make contact with patience-challenged drivers, you won't need the blotters, just a portable high-pressure washer to hose down what's left.[}:)]