minimum US curve radius
Hello Taurus,
The best contemporary practice is a curve no sharper than 12 degrees, about 146 meters, but a contemporary railroad would use such a curve only if there is no alternative. The sharper the curve, the greater the wear and stress on the roadbed and equipment, which translates into money spent on maintenance.
You can find the minimum turning radius of various types of diesel locos on line at The Diesel Shop,
http://www.thedieselshop.us/INDEXBLDR.html. This site has specifications, including minimum curvature, on models from the middle of the last century to date. Typically, these minimum curves run about 60 to 90 meters, if memory serves me. Some locomotives were designed to go around extremely sharp curves. I just looked up the EMD SW9, a locomotive designed especially for switching (shunting, I guess, in European parlance), and it was designed to negotiate 39 degree curves, about 45 meters in radius. But this is just the sharpest curve the locomotive alone could negotiate without suffering damage or throwing a tantrum and jumping onto the ties.
The real limiter is the couplings. In close built up areas there might be curves quite a bit sharper than 12 degrees, but special precautions would be taken when negotiating them. In some cases, cars would be uncoupled and pulled around such curves with chains. Special motive power might also be employed, such as the Pennsylvania RR's fleet of rubber tired tractors that rode on city streets while pushing and shoving boxcars to their cramped destinations. Obviously, the speed limits would be very low, probably lower than 10 mph.
Basically, if there is a profit in it, railroads will find to a way to get their equipment in position to earn it, so just about anything that does not defy the laws of physics is permissible. In fact, there is a school of thought that every model railroad should have one bizarre thing, but no more than one, for interest.
Happy hunting for info. The whole subject of railroad ingenuity is fascinating.
Bernie
PS: Several US railroads have sites that set out their specifications for curvature and track construction and geometry designed for customers constructing privately owned trackage for use by the railroad in serving them. You might try Reading Northern and the Union Pacific sites to start.