Running high speed trains on mixed mode rails (with commuter and freight traffic on the same lines) leads to one of two things: Either you slow your high speed

Running high speed trains on mixed mode rails (with commuter and freight traffic on the same lines) leads to one of two things: Either you slow your high speed train down (lowest common denominator) or you severely reduce capacity of slower commuter trains. This is why the HS2 project in the UK will free up so much local commuter capacity. By taking all high speed trains off the existing lines, you can run many more trains on them.