To add a preview schedule you’ll need to follow the steps to add an XML file here – https://www.bctransit.com/docs/gtfs/adding-gtfs-xml-files/
Once the XML file has been uploaded, add the link to the Upcoming Route/Note XML File Url metafield

Once this file has been added, you’ll need to set a preview date in the Schedule Preview Date metafield
This date will the date that the preview button will appear on the route-overview page.
In addition to setting the preview date, you will need to set a date for when the new preview schedule is meant to turn into the live schedule in the Upcoming Route/Note XML File Date metafield. On this date, the preview XML file will turn into the new live schedule.
When it is time for a new preview schedule, the old preview schedule XML link will need to be copied and pasted into the Route/Note XML File Url metafield
The cronjob will need to be ran after these changes, followed by the cache being cleared. See cronjob documentation here – https://www.bctransit.com/docs/gtfs/cronjob/