01-08-2016, 09:44 AM
Hi,
There is no scientific basis on which this is calculated, for UK mainline. It was historically 200 yards for colour light signals = 183m in metric and became rounded to 180m. It works so we stick with it.
In some areas the standard overlap length has been increased to 225m - I think this is where the higher speed pendolino trains run (West Coast) but I'm sure somebody on here (PJW?) can confirm/deny that. In any case it is always possible to have a longer overlap, perhaps because 180m lies in pointwork, or there is already a need for an track joint (or axle counter) close to 180m but too close to allow another joint, or just for reasons of economy. In the Paddington area the overlaps are regularly around 200m, and not uncommon to find up to 400m.
Occasionally a phantom overlap is used where the overlap is nominally part-way through a track. In effect this is used where the overlap tests a track clear, but not certain points within that track where they are outside the Phantom overlap length.
There is no scientific basis on which this is calculated, for UK mainline. It was historically 200 yards for colour light signals = 183m in metric and became rounded to 180m. It works so we stick with it.
In some areas the standard overlap length has been increased to 225m - I think this is where the higher speed pendolino trains run (West Coast) but I'm sure somebody on here (PJW?) can confirm/deny that. In any case it is always possible to have a longer overlap, perhaps because 180m lies in pointwork, or there is already a need for an track joint (or axle counter) close to 180m but too close to allow another joint, or just for reasons of economy. In the Paddington area the overlaps are regularly around 200m, and not uncommon to find up to 400m.
Occasionally a phantom overlap is used where the overlap is nominally part-way through a track. In effect this is used where the overlap tests a track clear, but not certain points within that track where they are outside the Phantom overlap length.