Hot rail tracks

In modern long welded rail sections the rail is cut and welded into place under tension to reduce the likelihood of distortion under expansion. Basically once in situ the new rail is cut slightly too short using laid down formulae to determine how wide the gap should be depending on the ambient temperature at the time of installation. Once cut the resulting gap is often only an inch or 2 but the new rail then needs to be stretched using hydraulic rams (referred to as pulling kit). When the rail is stretched an the resulting gap closed up it can be welded into place, the pulling kit holds the rail under tension until the weld is completed. After welding has been carried out the pulling kit is removed and the excess weld is ground away to leave a smooth continuous running surface.

Effectively it means all continuous weld rail (CWR) is deliberately slightly too short and under constant tension. After years of use the pounding of the wheel on the rail tends squash the rail and make it longer when this happens the risk of heat distortion becomes a risk so it is usual to 'restress' the rails by cutting out a small piece then closing up the gap to reweld it.

Owing to the way rails are installed under tension it is rare for new track to be laid in the extremes of temperature / weather. Ocassionaly after derailment damage it is necessary to intall new track at short notice but ordinarily it is planned to take place when it is expected too be neither to hot or too cold for stressing

This photo shows the pulling kit stretching new rails prior to them being welded under tension.

https://permaquip.co.uk/en/product-store/stressor/stressor-hsm70-with-flat-face-couplings-(ffc)/

Edited By Gas_mantle. on 28/06/2018 10:53:39

Edited By Gas_mantle. on 28/06/2018 10:58:19

I think the term stretching is perhaps used out of context because I would imagine that all the “stretching” process is doing is just pulling the rail into line, and effectively takes up any slack. You would need a really powerful piece of kit to actually “stretch” a section of carbon steel such as that used for rails, I may be wrong with my assumption but am sure they are just taking out any slack.

Dave W

Dave, the rail is actually stretched despite it sounding far fetched.

In a length of CWR (usually 600ft) stretching by a few inches doesn't take as much force as you might expect and is easily acheivable with pulling kit.

It's usual to retress after rails have been in use for a number of years, this is because the downward forces (and heavy braking in some areas) cause the rails to lengthen (rail creep) and therefore reduce the inbuilt stress. Restressing by removing a small amount of rail then pulling the gap to reweld is primarily about restoring the stress and not necessarily straighten out any slack.

**LINK**

Double skinned cabinets stop that

With the weather we've currently got, they could probably achieve sufficient extra stretch in a length of rail with a couple of crowbar weilding navys.

I've got the same problem with hot lines meself...

I thought modern welded tracks where allowed to move in their sabots?

I don't know if our "bullet trains" were affected by the heat

Continuous welded rail is mounted directly onto the sleeper without the old fashioned 'chair' fitting common in older installations. A small rubber mat under the rail bottom adds a cushioning effect as well as provide electrical insulation to the rails if the concrete sleeper starts to crack exposing strengthening wires.

Once the rail is clipped down it should be unable to move.

**LINK**

Thanks to Gas_mantle for exposing another interesting technology.

I knew that improving the wearing properties of railway line was an early and ongoing application of scientific metallurgy - they are not made of carbon steel. (More likely a manganese steel as these are harder and tougher.)

Before this thread I thought I understood how continuous welded rail accommodated temperature changes. Wrong again. It never occurred to me that rail could be tensioned to reduce the effect. There are some clever blokes about!

Dave

Most rails are hot rolled steel and for CWR they are typically in 600ft lengths.

Manganese is used mainly on pointwork, diamond crossings etc where there will be heavy usage / wear.

It's not often realised but in modern installations the rails are fed with a very low voltage but are electrically insulated from each other, the wheels of passing trains short circuit the rail to show the position of trains which can be controlled by a signalman 50 miles away.

Peter

Is that done using the "Wheatstone Bridge" principle that we learned about in school all those years ago.

Phil

The track circuits are actually nothing more than a simple low voltage circuit that trips a relay once short circuited by the wheels of a train, this relay can then be used to energise other circuits that are vital for the working of the signalling system. In old manual signalboxes before the introduction of track circuits the signalmen could look out of the window and see the postion of trains under their control but in modern installations the signalman usually can't see the trains under his control so the track circuits display vital information of the precise location of trains relative to the points and signals.

The new signalbox at York is still only partly operational but is planned to control the entire East Coast Main line from Kings Cross to the Scottish border. Didcot signalbox just west of Reading already controls London Paddington to Bristol. The intention is to control the entire UK network from 17 signalboxes over the next decade or 2.

When the old joke about leaves on the line crops up every autumn it does make the UK rail system sound a laughing stock but the general public don't realise it is a serious condition that affects the integrity of the signalling system and potentially can be dangerous. Modern rolling stock tends to be fitted with disc brakes so there are no brake shoes cleaning the wheel running surface at every brake application, over time a mulch of squashed leaves builds up on the wheels surface this can then start to insulate wheels so that track circuits no longer operate reliably. It's be no means rare for trains to 'disappear' off the signalmans display which of course can be very serious. Of course a build up of mulch on the wheels does affect braking capability and that in itself is a safety issue but in a lot of cases leaf fall delays are when more stringent signalling rules are invoked because of track circuit unreliability.

Incidentally, on track circuited lines (probably 90% of the current network) if a rail breaks provided the break is a clean fully open break the track circuitry is destroyed and the protect signal should revert to danger automatically.

To return back to the track, in the 60s / 70s it was decided to install steel sleepers on some none high speed lines, the idea was the were cheap and supposedly maintenance free, however in reality they were more of a nuisance than they were worth. As you can imagine steel sleepers are far more inclined to absorb heat making them a pain in the summer when rail expansion is a problem

Edited By Gas_mantle. on 30/06/2018 11:40:08

In the days of short rails joined by fish plates it was quite simple to introduce an insulated joint so that track circuits could be separated from each other. A simple low voltage supply and low voltage relay was all you needed albeit with carbon silver contacts to prevent contacts from welding to each other thus ensuring that gravity and a spring could be relied on to open the relay when a train passed or something failed. (Don't get me started on the philosophy of fail safe.)

But then the buggers in the M&E department said the S&T department couldn't cut their beautiful continuously welded rails to introduce electrical breaks and the track circuits became medium frequency AC devices that were a little hazy about exactly where they started and finished but essentially did the same job.

What really complicates matters is a major junction where all the lines have to be track circuited so that a lost wagon will always register on the track circuit no matter where it stops. When I left the railways 37 years ago it was becoming common on the continent to count the number of axles entering a junction and leaving the junction to ensure there were no nasty surprises for the next train.

Just to make life more interesting on electrified lines the running rails are used as an electrical return and so the delicate track circuits have to coexist with 100s of AC amps or 1000s of DC amps. (There may be a missing zero or two in there.)

Edited By Colin Whittaker on 30/06/2018 14:06:30