Although Google Maps doesn't do offline public transportation downloading, OsmAnd might help you here. It uses Openstreetmap data. Depending on the region, Openstreetmap data may be better, equal, or worse than Google Transit data. On the plus side, if you notice errors, you can go and correct it yourself and others can benefit from your corrections — all data are available under a free license. You can even go through the effort of checking in advance if the public transportation info for your destination appears up to date (although comparing with reality may be tricky), update Openstreetmap, and then download OsmAnd.
Many transit organisations have their own planning app. Some may work offline. Most probably don't.
NB: I have not yet tried OsmAnd for offline public transportation routing. Personally I plan my public transportation in advance, using online functionality only to find out about current delays, where no offline solution can possibly help.