They have to use trains to haul them around.

The 5 car trains in Switzerland
Furka: car train between Oberwald (Valais) and Realp (near Andermatt). You
only need reservations if you�re driving a vehicle heavier than 3,5
tonnes. An alternative is the Furka Pass road.
L�tschberg: car train between Kandersteg (Bernese Oberland) and
Goppenstein (Valais). You can�t make reservations. There is no alternative
road.
Simplon: car train between Brig (Valais) and Iselle (Italy). Reservations
aren�t possible. An alternative is the Simplon Pass road.
L�tschberg and Simplon combined: direct car train between Kandersteg
(Bernese Oberland) and Iselle (Italy). Reservations are required. They can
be made up to 5 hours before the train�s departure, but I recommend
booking your ride earlier than that. There is no alternative pass road.
Vereina: car train between Selfranga (Klosters) and Sagliains (Lower
Engadine). Reservations are not necessary. An alternative is the Fl�ela
Pass road.
(Oberalp: the former car train between Andermatt and Sedrun (Graub�nden)
has been discontinued in 2023.)
Car trains in Switzerland: the basics
How Swiss car trains work
You drive your car on the train. It has special wagons with platforms to
securely park on. Then you either stay seated in your car, or sit in a
passenger�s train coach: this varies per route.

Most car trains don�t have to be booked in advance. They work like toll
tunnels: you simply show up, pay (or use a pre-purchased ticket), and
drive on to the train. Reservations are only obligatory for the direct
L�tschberg-Simplon car train between Kandersteg and Iselle.

Why use car trains
If pass roads are closed, or if you simply prefer not to drive on passes,
car trains are a great way to reach the other side of the mountains.
To many travelers, it�s a fun experience!
It�s more eco-friendly than driving the pass road yourself.
Why avoid car trains
Many car trains pass long tunnels and lights will be turned off. This can
be unpleasant if you suffer from claustrophobia, fear of the dark, or fear
of tunnels. You can use your car�s interior lighting or a flashlight to
ease discomfort.
If you�re an experienced driver, travel when the passes are open, and
enjoy mountain rides, you may want to skip car trains.

https://www.myswissalps.com/travel/car/car-train/