Is building a test instance from a mix of both mock and real objects OK?
You would be testing the MapMarker more in isolation if you would inject only mocks.

Isolation is important for two things:


stability of test results
more explicit and direct feedback


If a bug would be introduced in the RailsMarker, this test would also fail, causing it to be less stable.
In case of this bug, there would by multiple test failures (also for RailsMarker and possibly other tests that use this class) making it harder to find the bug.

In general I would use mocks for any dependant object that has more logic than just getters and setters. That being said, it looks like the RailsMarker might qualify.