It was because the Steipler didn’t want to sit on the seats in the train since he thought there might be shatnez and stood the whole night instead of sleeping
IIRC, there was still more to the story; the Steipler had a unique schedule, staying awake for a full day, then sleeping for a long time, then staying awake for a full day. Before traveling on the train, he adjusted his schedule, stayed awake even more than usual, figuring to sleep on the train. When the chashash shaatnez kept him from sleeping, he ended up having gone days without sleep.