You're right. There is no way to do that. If you're in an historical situation where railroads are important, about the only thing you can do is pre-build rr's, limiting thier effect and then leave the railroad advance out of the game, making sure no one can get it. Then the settler types won't be able to build railroads.
If you need rail building, then the other way to limit the railroads is to limit the number of settler types. Make them available only through events and what you give a tribe at the start.
I am of course speaking in the abstract. What is appropriate to do will depend on your concept of the scenario.
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<FONT size="1">"There is no tiddle-taddle nor pibble-pabble in Pompey's camp." -- from Henry V</FONT s>