Length of incline to flyover

[34D]

Hello,

Can anyone suggest what could be the minimum length of the ascending part of a flyover that takes one railway line over another?

Assume that: whole area is flat, both lines electrified, and that at the bottom of the theoretical flyover trains will be doing 90mph. Also assume that there will be a a alternative route for freight (so the flyover is just for fast EMUs to use).

Thanks in anticipation

 

[jopsuk]

it'll be defined a maximum gradient and minimum clearance surely?

 

[Harpers Tate]

You need a clearance from the lower rail height to the underside of any structure of about 5 metres. Add another metre, say, for the rail height above ground for the lower rail and the thickness of the overstructure (eg steelwork, concrete beam) itself and you need to gain about 6 metres.

You can't assume that the train will be moving when it hits the gradient; it may need to stop and re-start for signalling or any other reason. Thus the gradient needs to permit a standing start. Railways rarely get much steeper than about 1 in 50. To gain 6 metres height with a 1 in 50 gradient would require a ramp 300 metres long.

 

[bluenoxid]

You need a clearance from the lower rail height to the underside of any structure of about 5 metres. Add another metre, say, for the rail height above ground for the lower rail and the thickness of the overstructure (eg steelwork, concrete beam) itself and you need to gain about 6 metres.

You can't assume that the train will be moving when it hits the gradient; it may need to stop and re-start for signalling or any other reason. Thus the gradient needs to permit a standing start. Railways rarely get much steeper than about 1 in 50. To gain 6 metres height with a 1 in 50 gradient would require a ramp 300 metres long.

And that is without any transition. Potentially Shaftholme and Hitchin offer a good idea of what is required.

 

[snowball]

I seem to remember that in a discussion of alternatives to Network Rail's proposals for the Ordsall Chord etc, Joseph Locke said that for a flyover to be used by freight he wouldn't recommend anything steeper than 1 in 100.

 

[Bald Rick]

If you are prepared to limit the flyover to EMUs, then 1:30 is acceptable. Freight needs 1:100.

 

[AM9]

In addition to the dimensions already mentioned, sometimes the line being 'flown over' can be lowered, either resulting from local topography or intentionally, particularly where there may be limited space to build a full length ramp, e.g. between two overbridges or near a station.

 

[Trog]

Perhaps more applicable to freight but also consider what the existing ruling gradient on the line is, or your new flyover could become a pinch point, reducing maximum train weight or by slowing down heavy freights capacity. Also alignment as a gradient on a curve is harder to climb than one on the straight.

 

[Ediswan]

Alexandra Palace may be near the limit. Constrained by geography into a small space. Mainly EMUs, but also ECML diversions, steam specials, some freight.

 

[The Planner]

Freight prefers to go under rather than over too.

 

[Hophead]

Freight prefers to go under rather than over too.

But would there not be more chance of the train being stopped on the incline? Which is to say, awaiting clearance to join the line, surely it would be preferable for a heavy freight to be heading downhill, rather than up? Or does it pretty much even out in the real world?

 

[NotATrainspott]

To passively provide for a future WCML-Crossrail link, the HS2 plans now include putting the eastbound GWML relief line on a flyover over the turnback/future WCML Crossrail tracks. The planned gradient is 1 in 27 on the western incline, as you can see on page 8 of this PDF.

 

[swt_passenger]

Examples for a recent significant flyover, used by passenger and sometimes freight, at Reading.

Heading west from level through the station.

36m 22ch to 36m 49ch at 1 in 93 rising
36m 49ch to 36m 76ch at 1 in 730 rising (relatively level over top)
36m 76ch to 37m 26ch at 1 in 85 falling

One feature of the Reading layout is that the feeder lines that take trains from Reading West to the relief side of the main station have a falling gradient until they cross under the main viaduct a couple of metres below local level and then they rise back up to level approaching the station, so the clearance is achieved by both raising and lowering the different routes.

So in principle you could halve the overall distance needed for a 'flyover' by gaining half the clearance in the route going over the top, and the other half of the clearance by lowering the lines going under. IYSWIM.

 

[Philip Phlopp]

But would there not be more chance of the train being stopped on the incline? Which is to say, awaiting clearance to join the line, surely it would be preferable for a heavy freight to be heading downhill, rather than up? Or does it pretty much even out in the real world?

It depends on location and track layout.

The benefit from a dive under is that it allows a train to get up to speed before the incline, and as most dive unders are sheltered, they tend to have good clean rail conditions which allow more power to be applied without wheel slip, giving a heavy freight train a good chance of climbing up the gradient on the other side of the dive under.

A fly over, on the other hand, requires the locomotive to do all of the work to start with, on rails which are generally open to the elements and possibly after a relatively low speed turn out, so it's starting as with the dive under from a low speed, but is pretty much immediately having to lift its train up an incline without losing power from wheel slip, which is more likely.

 

[34D]

Thank you guys. I shall do a diagram later or tomorrow

 

[Ploughman]

Hello,

Can anyone suggest what could be the minimum length of the ascending part of a flyover that takes one railway line over another?

Assume that: whole area is flat, both lines electrified, and that at the bottom of the theoretical flyover trains will be doing 90mph. Also assume that there will be a a alternative route for freight (so the flyover is just for fast EMUs to use).

Thanks in anticipation

Is this a college project?

 

[The Planner]

It depends on location and track layout.

The benefit from a dive under is that it allows a train to get up to speed before the incline, and as most dive unders are sheltered, they tend to have good clean rail conditions which allow more power to be applied without wheel slip, giving a heavy freight train a good chance of climbing up the gradient on the other side of the dive under.

A fly over, on the other hand, requires the locomotive to do all of the work to start with, on rails which are generally open to the elements and possibly after a relatively low speed turn out, so it's starting as with the dive under from a low speed, but is pretty much immediately having to lift its train up an incline without losing power from wheel slip, which is more likely.

I always thought coupling strength would be part of the issue, if you stop a rake of wagons on the summit with some hanging on the incline there is a possibility of splitting?

 

[Philip Phlopp]

I always thought coupling strength would be part of the issue, if you stop a rake of wagons on the summit with some hanging on the incline there is a possibility of splitting?

Day to day, it's getting freight stuck and causing all sorts of mayhem whilst a banking locomotive is sent for, but having trains divide and couplers fail is definitely part of the issue, trying to limit the potential for that to happen. I certainly wouldn't like to see Acton dive under being swapped for a fly over with the length and weight of the trains operating out of Acton yard.

I would normally say it shouldn't be an issue, freight wagons are all braked and there shouldn't be an issue with freight dividing when held on a gradient, but every single time RAIB have been out looking at freight train derailments or incidents, they've found maintenance problems, so yeah, I certainly wouldn't be counting on freight not dividing or the brakes working perfectly.

 

[edwin_m]

Day to day, it's getting freight stuck and causing all sorts of mayhem whilst a banking locomotive is sent for, but having trains divide and couplers fail is definitely part of the issue, trying to limit the potential for that to happen. I certainly wouldn't like to see Acton dive under being swapped for a fly over with the length and weight of the trains operating out of Acton yard.

I would normally say it shouldn't be an issue, freight wagons are all braked and there shouldn't be an issue with freight dividing when held on a gradient, but every single time RAIB have been out looking at freight train derailments or incidents, they've found maintenance problems, so yeah, I certainly wouldn't be counting on freight not dividing or the brakes working perfectly.

How much freight will actually use the diveunder? I was under the impression the really heavy stuff went into the yard so stayed moreorless on the level over the top of it, and the things doing the diving are mainly Crossrail and other passenger trains on the Up Relief.

On coupling strength it's surely a matter of simple physics, that it mustn't be less than the tractive effort of the locomotive(s)? Having said that I seem to recall that when the class 60s were first delivered a risk was foreseen that if the driver applied full power while the brakes were still releasing on the train, the result might be to pull out the coupling on the first wagon.

 

[swt_passenger]

How much freight will actually use the diveunder? I was under the impression the really heavy stuff went into the yard so stayed moreorless on the level over the top of it, and the things doing the diving are mainly Crossrail and other passenger trains on the Up Relief.

AIUI from the GW line diagram (that was doing the rounds a year or two back from the ORR site) that there is still a surface route for the up relief as well, so a freight running beyond Acton Yard on the up relief can stay on level track anyway.

 

[colchesterken]

D L R in London is great fun, like a roller coaster up down, underground, sharp turns
I love it