Why not have the shuttle train horizontally separated and not vertically separated. Just have a normal self powered electric carriage on a station siding, load with passengers and time it's departure to synch with the large train and link up, unload its passengers to the fast train, then load departing train passengers, uncouple, quickly move to a siding, and and slow to arrive at the next station. No fancy elevation, just extra sidings for stopping and starting the shuttle car.