I appreciate this coverage from Agency, but it got me to wondering if passenger trains had always just blasted through. The Official Guide for June 1941 has an answer:
Agency City, six miles east of Ottumwa, had a telegraph office. At 12:54 p.m. the Des Moines - Galesburg motor car stopped there on flag. At 5:15 p.m. the Chicago to Ottumwa coach-only local stopped on flag. At Ottumwa it was combined with the Exposition Flyer to Omaha. There were also at least three Burlington Trailways trips a day making flag stops there.
For fans of whistles, there were six express trains in each direction that likely shook the telegrapher right out of his or her chair as they rolled through. Not counting extra sections and the first troop trains...