I've played with this a fair amount and have determined there isn't a simple way to render Google Transit Layer onto a map I've created through Google My Maps. These are the solutions I've come up with which all lead to dead ends:
- Use the Google Maps API to render the Google Transit Layer and my Google My Maps map. The issue with this is I've read My Maps isn't accessibly through the API, and even if it was, I wouldn't be able to use the Google Maps phone app to view the two maps, I'd have to navigate to a page I'm hosting.
- I've found 3 stores of public GTFS feeds, none of which have the cities I'm looking into (but Google does). If they did have the cities I wanted, I did find Google's GTFS to KML development tools in Python. Then I would have been able to import the KML to create a layer on My Map, but no such luck.
- Use a 3rd party service such as Maps Channel to composite the two maps together. I can't seem to get this service to work, but it stands to reason that if it has access to My Maps that the API should allow access, but who knows.
If someone comes upon a solution to this which doesn't require manually creating a GTFS or KML file for a city or something equally labor intensive, let me know.