From a safety perspective we won't see high speed portions located within a freeway median or close to an active freight line without significant mitigations. I have heard FRA personnel express concern about the Roadrunner configuration in a freeway median (potential risk of intrusion by heavy truck, limited access for emergency responders) and about the recent light rail derailment in Denver caused by spilled coal from a derailed coal train.

In response to an earlier post, I'll note that a number of portions of France's TGV are on completely new alignments rather than superimposed on an existing route.