To stand on ski in the summer

Story overheard at the train going to Oslo:

When I was a little girl in Fredrikstad my father made a bet with us children.
He insisted that the train could stand on ski!
Not only that, but the train could stand on ski in the summer!

It sounded like complete rubbish, but he sounded very confident.
What was up?

One summer, going on a holiday with our car in the direction of Oslo, my father drove the car to a railway station we did not know. On the station a train was waiting.
The sign on the railway station said SKI.
My father pointed at the waiting train and announced: "Here you see, the train can stand on Ski in the summer! "