A pipeline doesn't need viaducts because it doesn't have severe restrictions on curves. It can follow terrain:

https://en.wikipedia.org/wiki/File:Trans-Alaska_Pipeline_Sys...

A high-speed railway needs to be flat, so it needs tall pylons to support it above uneven terrain:

https://en.wikipedia.org/wiki/File:SNCF_TGV_Duplex_Viaduc_de...

https://en.wikipedia.org/wiki/File:Millau_Viaduct_constructi...

Trains often need viaducts due to those restrictions but a multi span footbridge over a swamp is a viaduct as is the Disney monorail. My point is simply elevating a span is not that expencive. However, as you say high speed rail needs a vary flat track so cost estimates need to look at the actual path vs just saying 5m/km is rediculus without any justification.