Flyovers And Diveunders


A Flyover junction in railway terms is a junction where one of the diverging tracks climbs up on to a bridge to cross the main line, usually at an oblique angle. A Diveunder is the same thing, but with a tunnel instead of a bridge. In either case, the advantage is the improvement in the capacity of the junction that results from the elimination of conflicting movements where two tracks cross.

As a rule of thumb, it’s generally a good idea to have a height difference of at least 8m between two tracks that cross by bridge or tunnel, to allow plenty of height for rolling stock, ohle, and the structure of the bridge itself. You might be able to get away with a bit less if you are only using British stock and don’t intend to add ohle.

The gradients of the approach ramps are not too critical if your junction is going to be used mostly by multiple-unit passenger trains, e.g. on a metro system, but you should be careful if there are heavy freight trains using the line.

To keep the gradient to 1 in 50 (2%), you would need a 400m long approach ramp. In confined spaces, you can shorten the approach ramp if you can share the height difference between the two lines (e.g. one drops 4m while the other climbs 4m). Of course, you should design the signalling to avoid the danger of a heavy train having to stop and restart on or near the ramp.


Problems with Flyovers and Diveunders in Rail3D

If you try to build the approach to a flyover or diveunder in Rail3D, you’ll discover that autocutting isn’t very comfortable with closely-spaced tracks at different heights. This is reasonable enough - if you look at a real railway embankment, you’ll see that its sides are at a slope of 45 degrees or less. A flyover junction where earth embankments are used requires the tracks to move quite a distance apart as the height difference is created. As long as you respect this, it is possible to build embankment-based flyovers/diveunders. mrg Comment: It is possible to use auto-cutting on closely spaced tracks if you know how - see below


In a confined setting, where the tracks have to remain parallel, the solution for flyovers is to use a ramp made from concrete or steel. In Rail3D, you can simulate this by turning autocutting off for the rising track and using a linear scenery item for the ramp.


The bridge itself will also be a linear scenery item, of course. An interesting problem for real-life engineers is how to support a viaduct that crosses a railway at an oblique angle. In Rail3D you can get away with anything, of course but, if it worries you, one solution is to add a line of concrete trestles as a linear scenery item to the lower track.


Diveunders are more difficult. Because of the way the tunnel mouth works, a true tunnel must be approached by a normal-width cutting. The only way I’ve found of getting a diveunder to work in Rail3D is to cheat: don’t use a tunnel at all, but build it exactly like a flyover, i.e. using ramp and viaduct scenery items for the level tracks (autocutting off), and leaving autocutting on for the diving track.


Auto-Cutting closely spaced tracks

The key is an understanding of how auto-cutting works (which I will write up soon).

  • Autocutting generates terrain points spaced apart by the track link’s autocutting width
  • Autocutting never generates points within the footprint of an adjacent track.

So, in the examples at the top of the page, the cutting points would be 6 metres apart, the track would be 3.2 metres spacing (default values) and therefore cutting points would be within the footprint of adjacent tracks and therefore not generated.

To produce the above close spaced lines, I set the track spacing to 4 metres and the cutting width to 3.7, thus it is possible to fit in the cutting points between the track and you get the cutting (albeit steep sided, and it’s going to fall down the first time it rains)


loneaussie 13/02/2015 22:13:48