Railway Slab Track: The Pros and Cons for Rail Engineers

[@thepwayengineer]

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[@gljames24]

A high quality video with a low subscriber count is a rare find. I wish you luck!

[@thepwayengineer]

I really appreciate that! Thank you

[@krissp8712]

The algo's locked onto this channel. It's time to boost it up!

[@WhiskeyGulf71]

You missed out the huge cost & work involved to replace slab track, concrete over time does weaken, crack & fail, once water permeates the concrete the steel rebar inside begins to rust, as metal rusts it expands & this is what generally cracks the concrete open. Freeze jacking also contributes and Over time this gets worse until the concrete loses its structural integrity. Ballasted track is easy to lay, cheap & quick to maintain, you only need to clean the ballast from time to time, removing the dirt & small broken stones & lay it back down again. Slab track is better in a dry environment.

[@metropod]

I work for New York City Transit and the large majority of our underground track uses a distinct variation of this concept. Basically we still use wooden tie blocks (with every fourth or so a full tie) set into the concrete floor with a drainage trough down the middle. The drain also gives people who have fallen on the tracks somewhere to lay safely of a train is coming.

[@Helpoca]

The biggest drawback of slab tracks that you forgot to mention is the lack of ability to dilate. In every country with highly varied climate throughout the year, engineers has to account for dilation when designing train/tram lines (or bridges, or tunnel entrances, or pretty much everything), because at every hotter summer, if the tracks can't dilate, they'll bulge to a point where whole sections of the slabs has to be replaced due to cracks or fractures in the concrete as the tracks rip pieces off of it, and that doesn't only cost a fortune, but pretty much cripples tram or train lines for days. That is the main reason why the majorty of train/tram lines are still lie on ballast beds.

[@thepwayengineer]

The rails can be stressed to mitigate the thermal forces. I would imagine that expansion joints are also fitted in the concrete. But your point is a good one and I agree it is an issue

[@TheRailwayDrone]

By "dilation," do you mean expanding and contracting due to temperature? Forgive me but I'm still learning.

[@Helpoca]

​@@TheRailwayDrone yes, but the problem is more prevalent when it expands. The tensile strength of rail steel is incredibely high, so in colder climate, it's very unlikely that it'll break due to retraction. It's also has to be strong enough to bear the weight of a train (trains are very-very heavy), so by compressing it, it'll just flatten towards the base. That is the reason that if it expands due to high temperatures, it bulges up like a rollercoaster, but won't break. It bulges up, because the tie plates are keeping it in place sideways, and the ties are keeping it on the ground, so it moves towards the least resistance, which is up. And when it does, it takes everything with itself, including the tie plates, the fish plates, and sometimes the ties too. When it does that on a slab track, it takes the fasteners, and pieces of the concrete slab too. The problem there is that unlike ballast tracks, where you only have to replace a few ties and tie plates, with a slab track, you can't pour concrete into the holes, cracks and fractures the rail rips open, because that'd affect the overall integrity of the slab, so you have to replace the whole slab. So while on a ballast track, you'd be able to repair only one of the rails of the track, a few ties, and a couple plates, which can be done in a few hours, and can be done almost immediately, on a slab track, you'd have to remove the whole slab and put another one there, for which youd need a freight train with a crane and the new slab, which takes much more time and requires much more preparation. Obviously, it all depends on the situation and the severity of the damage, but generally speaking, if you don't build and maintain slab tracks properly (and almost no one does), you'll be up for a challenge fixing it if something goes wrong.

[@TheRailwayDrone]

@@Helpoca Interesting! I wonder how often Germany has the problems you described. I've seen many of their high speed railways using slab track.

[@Helpoca]

@@TheRailwayDrone I'd imagine german engineering applies to their railways too, and they sorted this issue out before it ever happened ;)

[@supermerill]

Also, from sncf reseau (french rail infrastructure operator): The ballast dampen, so it's less noisy. The lesser flex in the slab track makes the wheels (and rails) wear faster.

[@allangibson8494]

How the track is attached feeds into the noise issue. Spring clips rather than bolts are now common as are plastic buffer pads under the rail.

[@thepwayengineer]

It does have to be remembered that the whole track structure is a system when even the humble pads have a key roll to play. Changing any part has a wider impact

[@ClockworksOfGL]

The earliest train tracks were rigidly mounted, but they quickly discovered it was better to allow flexing.

[@mx338]

Riding on high speed concrete tracks is also very pleasant, it very noticeable reduces the shaking and noise.

[@felixonrails]

If you look at the Gotthard Base Tunnel, you can see another disadvantage of Slab track. It's insane work to rebuild in case of a derailment.

[@felixonrails]

And I agree, wonderful channel and videos❗

[@allangibson8494]

The opposite is also true as derailments are greatly reduced because of more tightly controlled track geometry…

[@thepwayengineer]

Both @felixonrails and @allangibson8494 make really valid points

[@thepwayengineer]

Thank you very much!

[@Ihwaz13]

One more advantage of slab track is that it allows higher speeds and the use of eddy current brakes. Both at high speed and when eddy current brakes are used the ballast would be sent flying, massively increasing maintenance, damaging trains and being hazard for anything close to the rails

[@thepwayengineer]

I will be honest, I have not come across eddy current brakes but would be interested to know more!

[@einereinar]

@@thepwayengineer As a starting point, the ICE 3 uses eddy current brakes on certain high speed lines, if that helps.

[@Sp4mMe]

An interesting aspect of this is that esp. in tunnels the fixed slabs aren't necessarily fixed to the tunnel. Instead they are on a mass-damper system. This handles vibration and noise issues. In some constructions, like parts of Stuttgart 21, it's done in such a way that the entire slab system can be lifted up to be accessed from below (to an extend) for future maintenance needs.

[@red_rassmueller1716]

Thanks mate. I really like your videos. Still confused by your viewcount though. You should have 10k at least😢

[@thepwayengineer]

I appreciate that! Thank you very much.

[@markfisher7962]

I wonder about rolling resistance. I imagine that ballast track would compress under the train's weight, so that the train was always slightly climbing as it traveled. I've seen asphalt roads compress ~2 inches as heavy mining trucks traveled along. The road surface then rebounded.

[@flexiblebirdchannel]

You missed the main drawback: a concrete slab is not stable (it will break if lifted at one end that may be miles away), it requires support from below, and below is earth and earth moves. Earth moves always, either it decreases in height because of organic decomposition and erosion, groundwater will wash out even far below the surface, and even whole landscapes move because mountains sink and earth crust moves. While ballasted tracks will follow this movement gently (except in case of earthquakes) by introducing slight curves and height varations, slabs will not. They break and build sharp steps that rails and trains can't follow. So slabs need adjustable support, no problem on bridge pillars or inside tunnel tubes. Otherwise only short distances can be used. It's like concrete highways vs. asphalt: The concrete highways need predetermined breaking points, otherwise they may occasionally break and introduce huge steps that crashes cars. Asphalt will follow ground movement at least to a certain degree. Concrete needs foundation that can be adjusted to earth movement. So concrete (slab railway or highway) are great on bridges and tunnels or for short distances, but unsuitable for long distances

[@markopinteric]

Hey, what about the noise? Slab tracks are much worse in terms of noise pollution from railway operations.

[@James_Knott]

There is some slab track going in right in front of my condo, with a new LRT line. They appear to be using blocks that the rails mount to. These blocks are laid on the reinforcing mesh, before the concrete is poured around them. This LRT is supposed to open early next year. BTW, I had never heard the term slab track until now, but I've certainly seen it in the subway for many years.

[@thepwayengineer]

Where is the LTR railway if you dont mind me asking? Thank you for sharing and I hope you found the video interesting!

[@James_Knott]

@@thepwayengineer That's LRT for Light Rail Transit. It's a new line going in right down the front of my condo property and will replace buses. It's in Mississauga Ontario.

[@DB-thats-me]

Side topic. I was having a discussion with a newly minted drain driver and I used the term I.P.Way. He stopped me and asked what mechanism that was. He was even more surprised to find out it was a who, not a what! Further investigation uncovered the fact he had NEVER heard the term ‘Permanent Way’! 😳😱🤬

[@Luredreier]

So what about changes in temperature changing the length of the tracks and/or its shape?

[@deano3580]

Ballasted track these days uses continuous rails without joints. It is correct that if this is not well built and well maintained then there can be a risk of buckling because of temperature changes. However, slab track is so heavy and the rails so well fixed that buckling is way less likely if not impossible.

[@thepwayengineer]

It is also common to use continuously welded rail that is stressed to avoid changes in length

[@TheAZPro-yi8bu]

Neglected to mention: ALL rails eventually where out. Replacement is very expensive.

[@James_Knott]

For the most part, the rails are open and held down with clips, just like on ties. The exception would be where the track crosses roads and the area around the rails is built up with concrete, to maintain a level road surface. This is the way it's done on a new LRT line that runs right in front of my condo and it has it's own right of way in the middle of the road.

[@johnawalker9261]

Misuse of the word impact.