The problem is the length of the parallel track. A high speed train does 200mph, if you allow 15mins for the people to transfer on/off the catch-up car then you

The problem is the length of the parallel track. A high speed train does 200mph, if you allow 15mins for the people to transfer on/off the catch-up car then you need them to be parallel for 50miles! And that's only with current trains, this system is only really necessary when trains get a lot faster.