Railway tracks?

Question

Now the railway companies have taken out the gaps in the tracks that made that distinctive noise when travelling by train, how does the track compensate for expanding and contacting as the temperature changes?

Answer 1

Though through engineering the amount of expansion and contraction the rail will handle is compensated to some degree, it is not a perfect scenario.

To the extent possible, when the rail is laid it is done so at a "median" temperature.  This value changes from place to place, taking into account the variable temperature extremes in the areas where the rail is laid.  For example, rail laid in hot desert areas are not lain at the same median temperature as one would find in mountainous areas where the temperature swings are more wide spread.

When necessary to heat the rail when being laid, there is a device on wheels that runs on propane that heats the rail to the desired temperature.

The rail is engineered in such a way as to minimize horizontal expansion through redirecting that tendency into lateral expansion.  Horizontal expansion problems are further reduced through the use of "creepers."  These are steel devices, shaped like a "candy cane", that are hammered onto the base of the rail on one or both rides of the metal tie plates that the rail rests on between the bottom of the rail and top of the cross ties.  These further reduce horizontal expansion, or "creeping."

During contraction in extreme cold, "pull aparts" are an every night fact of life.  The other day a question was posted here about it being seen that there were flames along a section of track.  This is a maintenance of way activity is called "burning a rope."  A length of rope is soaked in kerosese, laid next to the base of the rail and ignited to heat the rail so that the pull apart can be repaired.  An "angle bar" is installed as a temporary patch until the welders come along and weld the joint back together.

The flip side of this coin is when expansion is more that the rail can handle, creating "sun kinks," where a section of track will shift in the ballast creating a danger of derailment.  When these are extreme, it can lift the rail off the cross ties or kick them out of the ballast.  This is a guaranteed derailment.  In Europe some years back, this did happen and caused a train wreck of a passenger train with the loss of many lives.  I believe this was in France or Italy during a very atypical heat wave.

Answer 2

This is a very good question. Much better than so many that get asked.

Approximately every 2 to 5 miles (depending on where the section of track is) there is an expansion joint.

The ends of the rails are not cut directly across. They are cut in such a way that they are slightly less than half their width (normally about 5cm at the top) for a distance of about 80cm.

The abutting rail is cut in a similar fashion and the two 'tongues' are laid alongside each other so that the effective rail width at the top is again about 5cm.

The ends of each tongue are about 10cm short of the start of the full width part of the facing rail. It is this gap which takes up expansion.

Thus the train wheel always has a rail surface to run on, even if it's only part width for a short distance, and therefore the rider does not experience a bump when passing over the joint.

The rails themselves are held to the sleepers by special fastenings which permit longitudinal movement (linear expension) but hold them rigidly in all other directions (you'll be pleased to know).

You can sometimes see these joints at stations. They're identified by two short (about 2 foot long) parallel rails placed just inside the running rails. When you see one you will know exactly what it is - they're very self-evident. So there's no need to be confused by any of the other stuff that gets placed between the rails.

Incidentally, the continuous welded rail is not a modern invention. Most main lines and some branches have been laid with it for at least the last 40 years. It's just that railway lines last such a long time that you may not have noticed new track in your area for a while.

Answer 3

This track is welded together.  

To allow for temperature expansion, the rail is laid and welded under extreme tension.  

So when it's hot, the metal tries to expand but just reduces the tension, but this does not physically effect it's length.

Answer 4

The rails are no longer "butt" jointed but are cut to a very shallow angle so that two adjacent rail ends taper almost to a point and when placed together form an over-lapping sliding joint which although well greased, is held in place by special clips which allow the movement from expansion and contraction to take place without affecting the rail gauge.

Answer 5

Persumably you mean the expansion joints.

They are still there at each end of the piece of track, it would be impossible to get rid of them, when the track breaks or the expansion bars break you should see the rails jump!

Answer 6

Dover Soles and The Tank are dead wrong.  Railroads don't use expansion joints, except in extremely unusual cases, like making a transition onto a large bridge, where the bridge's natural thermal expansion would violently conflict with the rail's.  Even so, those expansion joints absorb only a few inches of expansion, a fraction of what normally occurs in rail.

How does track compensate?  It doesn't!

The rail TRIES to expand and contract, but it is not allowed to by the track structure.  Instead, it goes into compression or tension (like a guitar string).  

Now if it goes into tension too much, it breaks and you get a broken rail.  If it goes into compression too much, you get a "sun kink" (named because the heat of the sun is warming the rails, driving it toward expansion and thus compression.)

http://www.alaskarails.org/terminology/s...

http://www.usatoday.com/weather/news/200...

So there is a delicate science to laying welded rail at the right temperature so it stays within limits.  In winter, rail will never get colder than the ambient temperature, but in summer, the sun can heat the rail to much hotter than ambient.

Rail breaks can be reduced by high quality rail and good rail inspection (usually by vehicles with electronic sensors, like a Sperry car).  

Sun kinks can be reduced by making the track heavier, more rigid and more difficult to displace -- heavy ballasting, rail anchors or high-weight ties such as concrete ties, or simply slab track.

Now why are Soles and Tank wrong?   Let's do the math, shall we:

http://mathcentral.uregina.ca/QQ/databas...

What's the worst case for temperature?  -20F on a dead cold day, and +140F with the sun baking the rails. That's 160 degrees differential, 0.408672" per degree per mile = 65.38752 inches per mile.  In 2 miles, 13 feet.  In 5 miles, over 30 feet.  

So Tank proposes tapered expansion joints every 2-5 miles that absorb 13-30 FEET of rail movement!?  I don't think so!

Besides, that clever idea would encourage the rails to slide down the track, toward and away from the expansion joint.  That is something you do not want in railroad construction because the rail creep tends to damage the track.   You want track to stay put.

It's not that expansion joints don't exist, it's that they're used only in rare cases where the roadbed requires it.

http://www.bwg.cc/downloads/BWG_RZ_Schie...

And even so, they're designed to absorb only a few inches of movement, and not the several feet per mile that would ordinarily occur in rail.

Answer 7

Metal on  the rails expands and contracts with the temperature... there's not a h**l of a lot the engineers can do about the weather. So it's "clickity-clack" or take the bus.

Answer 8

I do wish we could have an English only version on here - Sorry, but Dover Soles is ABSOLUTELY right in what he says. In the UK the rail joints ARE angled along the length of the rail - they are NOT across it so therefore compression doesn't normally occur. Don't forget, we are NOT talking about USA or Alaska here - so the extremes of temperature described just do NOT apply.  Expansion is allowed for by laying the rails at a mean - average - temperature, and the sliding joints are made to compensate for expansion and contraction due to temperature changes outside of that. Admittedly, last year we had some trouble even WITH these sliding expansion joints so if no allowance had been made - by using butt joints - there is NO WAY that the track would have been held in place by the infrastructure - it would have just sprung right out of place and done untold damage.

If you STILL don't believe this, just come over and look for yourself. The very presence of these joints prevents all of those "sun kinks" that used to occur in hot weather but only do so now in the very extremes as we suffered last year. Don't forget, this is the UK site, so we talk about UK conditions.  American issues only confuse people over here who ask an innocent question only to find themselves in the middle of an unnecessary list of maths and links to prove them.