Page 11 has explained some difference in maintenance, that might result in a less predictable track geometry.
The difference in curve speed between Amtrak operation and tilting DMU operation in several European countries (Britain,
Germany, Spain) is huge - as shown on page 5. The high curve speed needs a waiver
by the authorities, that allows these 11.8 inches of unbalanced superelevation.
Different procedures in these countries assure, that the waiver is valid for this train on a certain track.
The existence of these waivers does not mean, that any safety authority on the whole planet would allow 11.8 inches in conjunction with common track maintenance concepts of certain US freight railroads.
The current limits in the USA, set by the FRA, might get replaced by less conservative values, based on existing knowledge and rational decisionmaking, But an operator of passenger trains should better expect, that operation on freight railroad track means at least "one inch below the construction limit", unless procedures are changed in favour of a more predictable track geometry.