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I would guess in a ring roller.

Basically a roller on top pressing down and 2 rollers on the bottom and you wind the metal through tightening the top roller to increase the curve.

Sorry to disappoint you.

If it makes you feel better though, the whales used to set the curves on boats used to be real whales.

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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.

Talking of ducks, some think that the spline weight 'ducks' could be the origin of the expression 'have your ducks in a row'.

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