Planar Network maintain a number of tools for converting various industry standards to GTFS. Where possible we maintain a public version of the feed itself on transitfeeds.com.
Data is supplied and licensed by Rail Delivery Group and is available directly through their data portal.
Download the latest GTFS feed: gb-rail-latest.zip
Coming soon.
We have an open source tool to convert SSIM flight schedules to GTFS but no data with a license to public the data in GTFS format. If you would like us to host any feeds please get in touch.