As well as converting to GTFS UK2GTFS has functions for
manipulating GTFS files.
gtfs_clean() Clean simple errors from GTFS files
gtfs_compress() Reduce file size of a GTFS object
gtfs_fast_trips() Find fast trips
gtfs_validate_internal() Validate a GTFS object (in
R)
gtfs_merge() merge a list of gtfs files
gtfs_split() Split a GTFS object
gtfs_split_ids() Split a GTFS object based on
trip_ids
gtfs_clip() Clip a GTFS object to a geographical
area