The General Transit Feed Specification (GTFS) is common format for exchanging transportation schedules and associated transit information. Specification is available at Googles GTFS pages. data.foli.fi provides both data package (gtfs.zip) as complete and also access to its contents through REST urls to ease it's usage on browsers.
Latest published package is always available at :
Contents of latest package are available also through REST api. Query interface returns GTFS tables as JSON formatted data with minor changes like using native datatypes instead of packages "string" attributes.
Exact descripton of provided data is available at Google's GTFS-documentation. Below is summary of published tables accompanied with simple examples.