Yes this line Signaled. As a matter of fact. There are two Vader signals off in the distance.
Would you mind explaining your statement a little for us non-railroader types?
Ballast up against the rail. Moisture. Electrical path between the rails. Shunts the track circuit. Lights the block on the dispatcher’s console. Trouble ticket to the signal desk. Call-out to the signal maintainer. Track inspector and signal inspector spend the night finding the shunt. Signal maintainer hogs out on hour-of-service and doesn’t do any work the next day. Trains delayed. Morning conference call brings it up. VP Transportation wants to know why hotshots didn’t make their cutoff this morning. Chief Dispatcher has already added up hours of train delay. Nasty phone call to the roadmaster from the district signal manager about his budget getting blown. Nasty phone call from the superintendent, subject: Fix your track-light problem or look for a new job.
S. Hadid
You’ve painted a nice picture, but I just don’t see it. Unless it is raining, perhaps. But if it were pouring rain, then you’d have that problem even without the ballast there.
The resistance of a moisure path from rail to rail because of ballast touching the rail would be so high that only a very sensitive meter would be able to measure it.
I’m going to show this to my signal engineer in the morning so he can snort. He spent 37 years in the field starting as a lineman and working his way up, and we both still spend plenty of our time tramping through the snow inspecting and troubleshooting signaling systems.
No, that would be mean.
What you’re missing is the relative current leaking through the ballast and ties instead of traveling down the rail to the track relay.
OK, do you have any idea how a track circuit works? Here goes, Signaling 101, D.C. track circuits.
A track circuit consists of a section of track (two rails), a battery and a current-limiting relay in the circuit at one end of the track section, and a relay in the circuit at the other end of the track section. The battery terminals are connected across the rails (for example, plus to the north rail and minus to the south rail), and the relay coil is connected across the rails at the other end. The current flows from the battery down one rail, through the relay coil, and back up the other rail and back to the battery – complete circuit.
The current is sufficient to pick up the relay. When current ceases flowing through the relay for whatever reason the relay drops out. That closes a secondary circuit which tells you something is out there in the track section, or perhaps a rail is broken, or the battery dead, whatever; it follows fail-safe principles. The secondary circuits are designed, in a simple case, so that when the relay is picked up you can obtain a proceed
Now you’ve painted a much more excellent picture and I can see the details much more clearly.
As I understand it, the problem is not so much that ballast may touch the rails here and there, but that ballast touching the rails for a signicant portion of the track section would cause very detectable current through the ballast - to the point where it would be difficult to adequately adjust the resistor to effectively detect track section occupation.
The current leak is always detectable. Remember, you have an awful lot of conductor (the rails) laying out there on the ground.
If the current path between the rails is good enough forget the resistor; you can’t possibly add enough current to pick up the relay no matter how many batteries you hook together. And pretty soon you have enough current out there that you’re frying trackmen, too.
In other words you want to make sure that:
between the rail resistance >> relay resistance >> wheel-and-axle resistance
S. Hadid
Gotcha!
Thank you for your detailed explanation. Technical, yet understandable. You should be a writer.
It would appear that the track on the left is more heavily used, based on the weathering patterns. But since CWR and concrete ties are much more expensive than wood ties, perhaps the difference in colorationis due to the right track being more recently upgraded. [?] Anyone know for sure?
MC and 1435 have pretty well covered the details (nice essay on signalling there, by the way!). Just one more minor point: why is there a ‘sag’ in the middle? (it isn’t really…). Why not? It’s just as simple to build a multiple use form with the lowered section in the middle as not, costs no more to pour (in terms of labour), puts the concrete where you need it (at the connections) but gives enough cover over the rebar in the top of the tie in the centre (where you need it to take the bending if the tie becomes centre-bound)… just good engineering desgn… for once…
Years ago Trains had a reprint of two articles “All about Signals.” I purchased it during the 70’s and it went into great detail of signals, interlockings, etc.
Mr. Hadid, I really appreciate your explanation. It certainly was a good explanation of how these systems work. I agree…you should write!
ed
To sum up a little bit: mechanical and civil engineers have to have one or more courses in the mechanics of solids. One of the things you are supposed to learn is how stresses, bending moments and the like propagate through a solid structure. The center of the tie will not encounter the same compressive loading as the regions closer to the rail interface, thus less concrete will be necessary. (One of the things you learn in the first solids course.) I suppose, without analytical support, that concrete sufficient to clad the longitudinal re-bar and provide enough stiffness to enable mechanical insertion of the ties would be much less than that required to support the downward force of the rail and the bending associated with that loading and the clips.
That said, remember that the optimist sees the glass as half full, the pessimist sees the glass as half empty and the engineer sees that the glass is twice the size it needs to be. If putting less material in the middle still works, then that is the prudent thing to do from an economic point of view. (Especially with the escalating price for cement these days.)
The concrete tie is prestressed, as well as precast, into a specific shape. They can be cast in molds as long as several hundred yards in length. The prestressing cables are passed through the end pieces that are inserted in the mold to create the length desired in the ties being cast, and the cables tensioned. The concrete form is shaped by the maker to cut down on sharp edges which would have a tendency to spall off in handling, as well as a mechanism to spread the loadbearing surfaces of the tie, and ultimately to save concrete.
Casting ties without prestressing would mostlikely create a very brittle product that would not last and would be much larger in size to be servicable on the railroad.
To me it appears as though the right hand track has been worked on recently. (New or cleaned ballast.)
One point not touched on in this discussion is that electromechanical relays have much higher pull-in currents than drop-out currents. This is generally a “Good Thing”, meaning that relays won’t chatter when the current is barely above the pull-in value (the logic circuit equivalent is a Schmidtt trigger). What this means in signalling circuits is that the rail-axle-rail resistance needs to be considerably less than the paralleled resistances of the relay coil and the stray current paths between the rails.
One way of getting around the problem of stray current between the rails is pulsing the track circuit voltage - where the relay
In existing track, yes, for an out-of-face job, concrete is more money. For new construction the price per foot – materials, labor, rock, OTM – is within $1 to $2 for concrete or wood. Wood ties have gone WAY up in price lately.
S. Hadid