Passing Loop - Gradients

Gradients

The design of crossing loops may have to be modified where there are severe gradients that make it difficult for a train to restart from a stationary position, or where the terrain is unsuitable for a normal loop.

One change would be to include runaway catchpoints on the downhill end, such as at Tumulla.

One oddity was Dombarton, New South Wales in Australia, where the crossing loop built to divide a long single line section on an extreme 1 in 30 (3.3%) gradient. The "loop" was built as a miniature zig-zag with a single track lower switchback on one side and a double track upper switchback on the other side, with a dive tunnel under the through track connecting the two.

A refuge loop built on the ruling grade on Cowan bank proved to be unworkable as the refuged train could not restart. This lesson was learned at Dombarton, where the uphill trains restart from level track. A refuge siding at Razorback halfway up the 1 in 75 ruling grade climb from Fish River to Cullerin also allowed trains to restart from level track.

A crossing loop on steep gradient may have catchpoints on the downhill end to reduce the impact of runaways.