Why do train tracks have gaps?

Question

Why do train tracks have gaps?

Answer 1

Rail used to be manufactured in 39' lengths.  The pieces were joined together by "angle bars" that allowed the two pieces to be joined together.  Later, rail was manufactured in 78' lengths, but still needing to be joined.

There are a couple of issues regarding jointed rail that has caused the wide spread use of CWR (continuous welded rail), aka "ribbon rail".  One factor is the rail joint itself.

That gap, even a small one, is subjected to constant pounding from very heavy equipment, all day, every day.  The result is a widening gap, that requires grinding and smoothing on a regular basis.

In addition, because of the length of the 39' and 78' rail joints, as a piece of equipment rolls over one, it dips to that side.  The joints are staggered, so at the next joint, it dips to the opposite side, back and forth, back and forth, with the amount of "dipping" increasing each cycle.  This is called "harmonic roll", and at its most extreme will allow a piece of equipment to overturn onto its side.  This phenomenon occurs the most at a speed of 15MPH.

These are the best arguments for CWR and they are sound.  But, there is a trade off.

The issue of expansion was touched on above, and is somewhat problematic.  CWR is manufactured is 1320' lengths and after being laid these joints are welded together, often creating a single piece of rail that can be miles long.  Expansion problems are mitigated to some degree in a couple of ways.

The rail is actually engineered and designed so that expansion will occur locally, on the cross section, left to right, if you were standing between the rails.  In addition, it is kept from excessive horizontal (longitudinal) expansion by the use of "creepers".  These are small steel devices, shaped somewhat like a candy cane, hammered onto the base of the rail, on either side of the tie.

But, in cases of extremely hot temperature, the rail around a curve is apt to pullitself up off the ties, spikes and all.  If not discovered by the time the temperarue cools, it can drop right back down and appear to be okay.  An AMTRAK train derailed near Corning, Ca., several years ago from exactly this situation.  These days, during summer extremes, most trains are issued track bulletins warning of heat restricitons resulting in having to operate the train at reduced speed.

The flip side of this, of course, is extreme cold.  In this scenario, the rail contracts to the point where it simply pulls apart.  Not a gap of feet, mind you, 2" or less.  Maintenance of way makes the scene and angle bars it back together, and later a welder will come along and weld the joint.

The latter is not nearly as serious of a problem as the heat scenario.  Rail that has pulled apart will knock down the block signals so any trains passing over will be at restricted speed.  When the rail buckles, however, it stays in one piece and the train comes right on along at whatever its maximum authorized speed is.

Answer 2

The gaps allow for expansion when they are heated by the sun..  Besides if there were no gaps you would fall asleep!! No bumpity bump...

Answer 3

I think you are referring to the joints between sections of rails on a track? That's how it used to be built.  We are moving towards more and more continious welded rail and you don't hear those click-clacks under the wheels as often.  I really liked Hoghead's elaboration.  It amazes me that we can lay down as nice a stretch of 40 mph track on CWR as you could want and some day in August it will have a couple of serious sun kinks in it.

I wonder what it costs to maintain track.

Answer 4

Reckon Hoghead  knows what he is talking about, He must work for Railroad.

Answer 5

Thermal expansion allowance

Answer 6

Mainly heat expansion

Answer 7

in summer due to the contraction of rails plase will be left

Answer 8

To allow the wrong type of snow to get into the gaps in Winter in order to shut down the railway so that the poor suffering commuters suffer even more whilst those barsts at British Rail can have mugs of steaming hot coffee and read all the papers that the poor suffering commuters have read and re-read whilst enduring the long delays.

Answer 9

They have gaps to allow for expansion and contraction. In the summer time the tracks will expand or get longer and in the winter they will shrink. Of course the amount of expansion is very small but if the tracks were butted together and the temprature was high enough to cause the track to expand, the track could bow and may be enough to cause a derailment. In the winter the track would shrink and cause a click as the wheels pass over it. Make any sense?

Answer 10

When the track iron heats up or cools down, it's lenght changes. Longer when hot and shorter when it cools. If there isn't gaps then the rails would warp at their weakest point and the two rails would not remain parallel and cause the train to derail.

Answer 11

They are called "expansion joints".

Rails are made of metal and metal expands as it is heated. Unless the rails are pre-tensioned, then as the metal heats up, without the expansion joints, the rails would buckle. Whereas the degree of buckle might not be readily visible, it would be sufficent to cause a derailment of a train passing over them.