With continuous (welded) track, there must be some incredibly long pieces of steel being used by railroads. Other than switches, and engineering restraints imposed by expansion and contraction, railroad tracks could be unbroken for hundreds of miles.
What is the longest contiguous length of track?
You mean welded together, or a rail line that can be driven on from end to end without stopping the train?
If the latter, there’s a line well over 1000 miles long across the Australian outback. The longest straight stretch (no curves or anything) is 300 miles long (in Africa somewhere).
<semi-hijack> Since track gangs have to lay new track to replace the old, wouldn’t welded track be a a pain to change? I guess they could just call the same guys that welded it to cut it… </semi-hijack>
CWR is generally welded up in lengths of from 400 feet to about a quarter mile at the point of manufacture (or a central rail yard), then shipped to location on specially-designed rack cars. The rail is flexible enough that a loaded rail train can pass through normal mainline curvature without problems. Once the rail has been spiked down, it can then be welded to greater lengths if desired. IIRC, however, you would be unlikely to find single continuous sections longer than a few miles.
On a signaled line, insulated joints are necessary to electrically isolate each signal section or “block”; in practice these are spaced from 1 to maybe 5 miles apart, depending on the railway and traffic levels. In addition, there is a need to include occasional joints to allow relief from temperature stresses, which would tend to kink or pull apart the rail. Perhaps there are some very long individual sections of CWR on unsignalled lines, but I doubt it.
Yes, replacement of welded track is a pain, but track doesn’t wear out very fast. Wheels do, especially as they click across the joints in non-welded track.
The railroads figure that welded track ends up cheaper. (In the long run, to get back to the OP.)
And for the heck of it here’s the marketing term:
“Ribbon Rail”
If you page through advertising of the 1950’s the term would be used to describe the smoother, quieter ride.
Welding track ends to each other eliminates the little gap the wheels have to go over, and allows for a higher maximum speed. For instance, the TGV lines in France (the world’s fastest train) are welded continuously. Once segments are put in place, the ends are field-welded together and the weld beads are ground smooth. There’s no problem with expansion and contraction as long as the connections to the ties are close enough together - the thermal loads are taken as longitudinal stresses in the rails themselves.
The railroads in the US have special locomotives available with grinding wheels mounted next to the tracks. When a section needs to be refinished, due to a flat area being worn on the top that increases rolling friction, they’ll send the grinding locomotive through to restore the crown. I’ve never actually seen one, but I understand they make a helluva racket. Of course, eventually there’s not enough material left to regrind, and the tracks have to be relaid.
I’ve seen one of these (in Illinois) and you are certainly correct in saying that it makes “a helluva racket.” Not being a railroad person myself, I thought something was seriously wrong when I saw it. It was big blue contraption, with showers and showers of sparks flying from underneath it in every direction. Quite a sight indeed.
Makes me wonder how much “longitudinal” expansion we are talking about? If the entire stretch of track is welded, wouldn’t the expansion be measured in feet?
How are the rails laid on the TRANS-SIBERIAN RR?
I assume that due to the great seasonal temperature changes , they could not be welded.
Also 9regarding the TSRR): are they still using the original rails, laid in the 1890’s?
The rails don’t expand because the ties restrain them. They go into longitudinal compression. On cold days, they go into longitudinal tension rather than shrinking.
That’s no problem IIRC as length doesn’t enter into it. If a piece of steel, 10 centimeters long, would expand or shrink, say, 2 millimeters because of differences in temerature, so would a 10 kilometer long piece of steel.
No, that’s not right. A 10km piece can be thought of as 100,000 of the 10cm pieces stuck together. Each piece shrinks by 2mm, so the whole thing shrinks by 2mm x 100,000 = 200 meters! But this requires a 200 degrees C of temperature change.
Floater, we are trying to fight ignorance here. Please.
For a given temperature difference an object will expand or contract a given percentage of it’s own dimension.
The answer has been given above already. The rail tries to expand or contract longitudinally but cannot and, so, goes into tension or compression.
I will add that this makes it contract even more in the othe two dimensions.
It would be interesting if someone can calculate the stress on (say) a length of beam or rail which is not allowed to expand or contract longitudinally, if it begins at 60F and swings 55F in each direction. Would the stress be similar to the stress of a beam in a building?
So if expansion and contraction aren’t issues, only signal areas and switching require breaks in rails. So, again, what is the longest contiguous track? Must be quite long – hundreds of miles.