Press the i button in the top right hand corner of the card. Select 'Transportation Mode' from the expander. Select from Driving, Public Transport, Walking, Cycling etc.

Excellent, thank you so much for this. "Next train leaving in <x> minutes" is sweet.

I always use public transportation in Boston and configured it to show directions as such the first day I had it last fall. It's probably more useful if you use it this way because there are usually multiple possible routes to take and it gives a very good indication of when to walk out the door.