Track on bridges

[Crepello]

My interest was prompted by the January 2020 derailment of a freight train at Wanstead Park in East London, a line I travel over regularly (the Goblin)

The accident report has concluded that rotten longitudinal timbers were to blame, as well as an issue with one wagon.

I'd like to know why on some bridges timbers are used to fix the track in position, whereas on others I've noticed on this line, normal ballast is the method.

Thanks in anticipation.

[LMS2968]

Not really my subject area, but I think the weight of normal ballasted track has something to do with it.

[Wickham Green too]

........ and the structure of the bridge : a brick or stone arch will normally ( always ? ) have ballasted track - but it's more complex to keep ballast in place on a steel bridge ( and keep the steel accessible for maintenance ) so waybeams are often used. 

[Siberian Snooper]

Bridges with ballast troughs are deeper than those with way beams and this impacts on the amount of clearance under the bridge, as well as the amount of material required in its construction.

 

 

[Edwin_m]

The structure must also support the extra dead weight of the ballast.  

 

Nevertheless, Network Rail's preference is to have ballasted track over bridges where possible.  

[Mark Saunders]

44 minutes ago, Edwin_m said:

The structure must also support the extra dead weight of the ballast.  

 

Nevertheless, Network Rail's preference is to have ballasted track over bridges where possible.  

Where there is a way beam there is the  almost certainty that the ballast will compact and need to be lifted and repacked more often rather than ballasted which can be tamped over the approach and structure!

[Wheatley]

The tamping issue raised by Mark can manifest itself as a viscious kick as you go over the bridge, the track on the waybeams is fixed rigidly to the bridge whereas that either side can move a bit on the ballast. 

[Bomag]

 

For gravity structures, like arches, the weight of ballast is not an issue; in fact it is a potential benefit as live loading is going to be a lower percentage of total load. This reduces strain (movement) as trains travel over the bridges; for the same reason the impact on fatigue on structural members is less. Arches are however less efficient in terms of mass of the structure as a percentage of the capacity.

 

Something more structurally refined e.g. Forth Bridge. is much closer to the limit state. For instance the dead weight of the cantilevers at each end of the bridge are not heavy enough to stay on the bearings when the design train was on the relevant suspended span, so there there is 900tons of  kentlage in each end tower. If you had ballasted track instead of waybeams, the support caissons would be only just poking out the water.

[Trog]

I would agree with Siberian Snooper, this is usually all about construction depth, and the head room under the bridge balanced against the height needed for the embankments either side of the bridge. A bridge with long timbers or waybeams if you are talking about the Southern will usually have a smaller dimension from rail head to bridge soffit than a ballasted bridge.  This can give a big saving in construction cost when you are building a railway by allowing lower and hence narrower based approach embankments, and a smaller saving in the cost of the bridge its self as it can be a little more lightly constructed as it is carrying a less dead weight. This saving however comes at the cost of a future maintenance liability.

 

On ballasted bridge decks you will often see that on those with shallow ballast the track will have hardwood sleepers over the bridge, or if the ballast is somewhat shallow, the shallow depth but wider than standard EF or EG type concrete sleepers.

[Edwin_m]

11 hours ago, Bomag said:

 

For gravity structures, like arches, the weight of ballast is not an issue; in fact it is a potential benefit as live loading is going to be a lower percentage of total load. This reduces strain (movement) as trains travel over the bridges; for the same reason the impact on fatigue on structural members is less. Arches are however less efficient in terms of mass of the structure as a percentage of the capacity.

 

Something more structurally refined e.g. Forth Bridge. is much closer to the limit state. For instance the dead weight of the cantilevers at each end of the bridge are not heavy enough to stay on the bearings when the design train was on the relevant suspended span, so there there is 900tons of  kentlage in each end tower. If you had ballasted track instead of waybeams, the support caissons would be only just poking out the water.

Masonry arch structures are essentially over-engineered by definition - the amount of material needed to create the structure will bear huge loads before getting anywhere near its crush strength.  However metallic structures, which are the type that may have waybeams, are designed to a specified load and will probably require strengthening if the load increases beyond that (which might happen if converted to ballasted track).  They are also prone to corrosion, and ballasted track can make it difficult for this to be detected during inspections.  

Edited December 6, 2020 by Edwin_m

[Crepello]

Thank you guys. That's a lot clearer to me now. The particular ballasted bridge that prompted my query is at Leyton Midland Road Station; it's a steel bridge supported by pillars in the street below, which presumably support the weight.

[Wheatley]

The dangers of converting waybeamed bridges to ballasted decks without allowing sufficient access for future maintenance are demonstrated by the collapse at Stewarton a few years ago: 

https://www.gov.uk/raib-reports/derailment-of-a-freight-train-near-stewarton-ayrshire

[Reorte]

On 06/12/2020 at 20:51, Wheatley said:

The dangers of converting waybeamed bridges to ballasted decks without allowing sufficient access for future maintenance are demonstrated by the collapse at Stewarton a few years ago: 

https://www.gov.uk/raib-reports/derailment-of-a-freight-train-near-stewarton-ayrshire

As an aside it looks like the earliest Streetview picture of that bridge was when the replacement was being built.