The curve radius needs to be around 23,000 feet (4.35 miles) or larger for 200 mph. or 13,000 feet (2.46 miles) for 150 mph. For comparison, a one degree curve (5730 feet or 1.085 mile radius is fine for 79 mph to 90 mph.
What underbalance are you using, George? Using a fairly routine 5" superelevation (I believe they can go higher than that, but I'd have to check my criteria), for 200mph I'm getting an underbalance of less than two inches. Even a heritage baggage car is good for at least three.
They would have to be big ol' curves, though, no doubt about that.
Yes, at 23,000 feet radius, 200 mph, the combined SE plus unbalance would be 6.96 inches. However, . . .
For the sake of rail wear, comfort and particularly comfort over a wider range of speed, you really want to keep the superelevation to 4 inches or less and the unbalance to 3 inches or less. Where the space is available, you should be playing with 3 inches and 2 inches if you can, but that is in the luxury realm a lot of the times.
When thinking of curves in degrees turned per 100 feet of length, speed in mph and super in inches,
the SE, combined actual and unbalance = 0.0007 V^2 / Degree of curve
When doing it in radius measured in feet, speed mph and super in inches, the forumla is:
SE, again combined = 4.0 V^2 / R
For those playing in the metric world, meters, km/h and SE in mm,
SE, again combined = 11.8 V^2 / R
To get radius, you churn this around to, in feet, mph, inches, so you get:
Radius = 4.0 V^2 / SE
or
Radius (meters) = 11.8 V^2 / SE (millimeters)
plugging 200 mph and 7 inches into this gets 22,857 feet, so round this to 23,000 feet
at 150 mph, and 7 inches, get 12,857 feet, call it 13,000 feet.
You can really go safely as far as 6 inches of superelevation and 4 inches of unbalance, but rail life on curves like this goes through the floor, and if the train ever has to stop, the passengers slide off onto the floor, so the best thing is to work with 4 and 3 or less and keep the higher numbers in your back pocket in case of emergency. Some systems allow the superelevation to go up to 180 mm = really close to 7 inches.
At these high values, you can run 200 mph around a 16,000 feet radius curve, but it is far better not to. In fact, the current European standards say at these high speeds unbalance shall be no less than 80 mm, which is just over 3 inches.