A transfer between Stuttgart Airport and Cologne Bonn Airport covers 364 km in around 3h 45. It exists for the day the connection breaks — when the replacement flight leaves from a different airport and getting there is suddenly your problem rather than the airline's.
The pickup is driven by your inbound flight number: the driver watches the actual landing time and waits 60 minutes free after touchdown, which is what a diverted or delayed arrival needs. At the far end the drop-off is at the correct terminal for your departure — Cologne Bonn Airport has 2 and the wrong one costs a shuttle ride you do not have time for. Tell us the departing flight at booking and the schedule is worked backwards from its check-in deadline rather than forwards from a convenient departure. We run the same journey to Düsseldorf Airport, priced the same way.
Luggage stays in the car for the whole 3h 45, which is the single biggest practical difference from any rail route: nothing is lifted onto a train at Stuttgart Airport, off it again at the interchange, or onto a second one for the run into Cologne Bonn Airport. If the airline has rebooked you and is reimbursing ground transport, the invoice shows the route, the date and the fare in a form an airline will accept.
What you are committing to at 3h 45
If you have read the comparison above and still want the car, this is what you are committing to. One driver and one vehicle for the whole distance — no handover, no change, no platform — and a comfort stop wherever you ask for one, which on this length most people use once. The reason people choose it at this distance is almost never comfort. It is that across 3h 45 no gate closes on you and there is no second boarding pass to lose: the deadline you are driving towards is the only one left in the day. Your luggage is handled exactly twice, once at Stuttgart Airport and once at Cologne Bonn Airport, and the rebooking calls you still have to make are easier from a back seat than from a platform.
Checked bags are the whole argument
With cabin baggage only, the rail route between two airports is an inconvenience. With checked bags it is a different journey altogether: every interchange means lifting cases onto a train, off it, up a staircase when the lift is out, and along a platform to the right carriage. A car takes the bags once at Stuttgart Airport and gives them back at Cologne Bonn Airport. A saloon holds three large cases, a van takes seven passengers with luggage to match, and if you are travelling with sports equipment or instruments — the things that make a rail change genuinely hard — say so at booking and the right vehicle is sent.