[Peter Wilding]

Peter Wilding  asked the Naked Scientists:
   
Hi Chris,

I work in SANDTON, Johannesburg, and have been travelling past the construction of our new rail commuting system for the past 2 to 3 years. In the past 6 months the rails/tracks for this system has been stored near the roadside, which I can see, travelling past every day. The lengths are approx. 15 to 20 meters long (?),  Â and made of specially cast steel, I am told.

As no tracks run in a straight line, and all over the world there are undulations and bends, I have frequently asked the question and no one has been able to give me an answer, as to how are the rails / tracks shaped or bent to accommodate the inconsistencies of the landscape through which it needs to be laid? This is specially cast steel, possibly 100mm to 150mm thick,  
 Â 
Regards,

Peter Wilding
 Â 
What do you think?

[LeeE]

Track sections that are only 15 to 20 metres long are actually rather short these days and Continuously Welded Rail sections, up to several kilometres long are the norm and may be transported to site in lengths of up to 0.5 - 1km.  See:

http://en.wikipedia.org/wiki/Continuous_welded_rail#Continuous_welded_rail

The individual short sections of rail are not bent to follow curves but the outside rail may be heated to expand it and close any gaps prior to welding on tight curves.

[Madidus_Scientia]

Heating the whole rail at once can cause havoc; check this out



http://www.news.com.au/adelaidenow/story/0,22606,24975204-5006301,00.html

[daveshorts]

Also of course railway lines go approximately in straight lines. The curves are very gentle. I expect that before the continuously welded track most of the corners were taken in the joints between the rails. Though you can probably bend a 20m piece of steel a cm or two using a large crowbar.

[Geezer]

The rail is sufficiently flexible to follow the curves and countour changes. Rails can even be transported in long sections on flat rail cars without derailing the cars as they follow the curves in the track. The ballast around (and sometimes on top of) the ties/sleepers holds the track in position.

Long welded rails are actually pre-tensioned with hydraulic tensioners when they are laid to prevent them from buckling at high temperatures.

BTW - On track that uses short sections of rails that are bolted together with joiners (fishplates), in the UK the joints are positioned opposite each other. In the US, the joints are staggered so that they are as far apart as possible. As a result, trains in the UK have a completely different rhythm from trains in the US. Does South Africa follow UK practice?