I've not noticed any further upgrade option on them. The only additional item connected to railways I've noticed was a civ research item called national railway or something like that, I seem to recall it didn't improve performance but did something to improve commerce or something like that. Not something I considered worth worrying about, I only researched it because in the first campaign play through I was researching the entire civ tree just to see the impact of everything, I'm doing the same on the military side of the tree as imperial this time for the same reason.
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