Comment by bell-cot
4 hours ago
Not an expert, but...
As Wikipedia partly mentions, Shay designs were very well suited to low-speed operation on dirt-cheap (often temporary) tracks. That generally meant narrow-gauge, light rail & iffy roadbed, tight curves, and steep grades. Low-speed-only was the only sort of operation you'd want to attempt - tracks suitable for higher speeds would have cost far more, but provided minimal benefit for logging, mining, and other Shay use cases.
A bit fuzzier, but I also recall Shays being considerably more easily maintained (vs. conventional steam locomotives) in situations with minimal facilities, equipment, and trained staff. Not sure how much that was locomotives being designed to the customers' needs, vs. being intrinsic to the Shay mechanical layout.
And a bit of context - a century or so ago, modern heavy road construction equipment and heavy trucks mostly didn't exist. Vs. railroad tech did. So for logging, mining, and many situations where we'd say "d'oh, just build a road and use trucks" - they just laid some RR track and used that.
It is indeed wild that the easiest way to do large-scale logging in those days was to slap together some tracks and roadbed and run trains up the valley. And then tear it all down again when it was time to move to the next one.
The railway is the historical norm, roads as we know them needed rubber tyres to really 'get traction'. This was a late innovation compared to rail.
Rails for high speed with safety, ride comfort and advanced signalling to maintain a scheduled service is a very different proposition to some temporary rails to get to the front line of a war, a new coal seam or forest 'port'. The latter is much easier compared to a road but now we have big machines for the latter, not the former.