The system for applying railway loads within Archie-M is a little complex because it was added in a hurry. There are a number of separate isses that must be dealt with:
1) On a railway, the two wheels on an axle are considered to carry potentially different loads as a result of centrifugal effects etc. On roads, these effects are effectively eliminated by the distribution rules. It is therefore necessary to consider single wheels rather than axles.
2) The single wheel is distributed to a patch 800mm wide under each of three sleepers. The distribtuion to sleepers is done in the load file. The 800mm width is created by representing the wheel as a very short axle 500mm long. with a 300mm patch under each "wheel" that creates a patch 800x300 which is close enough to a sleeper patch.
3) The catual distribution is different. There is a side slope of 1:1 instead of 1:2, and there is no addition (the 1.5m in highway loading) This is dealt with by choosing railway distribtuion from Tools/Set Distribution Mode.
4) With an wheel load considered instead of an axle, the maximum distribution represented by the lane width must be reduced. The conservative approach is to assume a lane width equal to the distance between load points on the rails (usually assumed to be 1500mm. You may increase this if the rail concerned is more than 750mm from the edge of the bridge or from the nearest rail of the adjacent track.
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