That's not a Dll version. It's the archive header.
Suppress it by using archive flags it:
void save_schedule(const bus_schedule &s, const char * filename){
    // make an archive
    std::ofstream ofs(filename);
    boost::archive::text_oarchive oa(ofs, boost::archive::archive_flags::no_header);
    oa << s;
}
And remember to do the same on restoring, of course!
void restore_schedule(bus_schedule &s, const char * filename) {
    // open the archive
    std::ifstream ifs(filename);
    boost::archive::text_iarchive ia(ifs, boost::archive::archive_flags::no_header);
    // restore the schedule from the archive
    ia >> s;
}
See also