Measure the car length in inches (this works for any scale). Multiply the result by 3. That is your new minimum radius (and I'll bet it's not 18").
The more we demand highly detailed rolling stock, and the more manufacturers deliver, the more we will be confronted with having to use the "3X" rule-of-thumb for minimum radius.
North American railroads, in order to be profitable, must continually work at improving efficiency. Since the beginning of North American railroading, locomotives and cars have continually increased in size. On the prototype, many tunnels, bridges, and curves have had to be rebuilt at enormous cost because they couldn't handle the ever-growing size of rolling stock. Yet the costs in improvement to the right-of-way are quickly repaid by running longer trains with bigger cars.
In HO, 18" was a fairly successful minimum radius for freight cars and smaller steam engines in the 1950s. The cars weren't as detailed, and couplers were crude and over-scale. Even then, full-scale passenger cars couldn't go around 18" radius curves, so manufacturers made "shorty" passenger cars that were a scale 60-70 ft instead of the full 80ft. The "shorty" passenger cars could do 18" radius curves.
50 years later and 60-90 ft long cars are the norm on the prototype. Yet we still insist on using 18" radius curves that were marginal for some 1950s prototypes, and wonder why the things won't stay on the track.
An interesting fact - a model of a modern 80ft car in N is just as long as a 40ft box car in HO.
just some thoughts of mine