But a ring roller produces curves of constant radius, and for the railway application it would have to be absolutely enormous. From a quick DuckDuckGo search,

But a ring roller produces curves of constant radius, and for the railway application it would have to be absolutely enormous. From a quick DuckDuckGo search, it looks like they basically are just laid out the same way the "splines" from the article are: the curves are gentle enough that the steel is not very hard to bend.