The success story of the Frankfurt airport to Koln high-speed rail: Built between 1995 and 2002....it cost around $7 billion to complete. Today? It runs once per hour. The ticket is around $110 (this was the cost in 2006, it may have gone down) for a one-way ticket between the Frankfurt airport and Koln, 110 miles in distance.
It’s a sixty-minute ride...where the normal ICE train could do the the job in roughly 2.5 hours.
How many thousands of people ride the train daily? Well...it’s mostly businessmen because of the cost. At some point in 2006, they were reporting a typical training leaving Frankfurt for Koln had around thirty people onboard. The entire train consisted of two cars. I would imagine they could have held roughly eighty people on the two cars max.
As for paying off the billions invested? Zero chance. It pretty much killed off all chances of a second future high-speed rail system in Germany.
It only makes sense if you intend to run a fast line from some airport to the nearby town (say twenty miles max). Otherwise, it’s a total waste of funds.
It’s actually cheaper to fly between cities in Europe than to take a train.