When looking for driving directions on Google Maps, it’s possible for the user to click on a link to Add Destination. By clicking on this link, the user can add another destination to their trip. For example, say you wanted to start in San Jose, travel to Mountain View, and then finally travel to San Francisco. This feature will let you obtain directions for all phases of the trip instead of having to go through this flow for each phase separately. A pretty nifty feature. 

The interesting thing is that on Google Maps, it’s not possible to do this if you select public transportation as your method of travel: 

Not sure why this is. From a functionality stand point it’s definitely possible for the service to be able to offer this type of user query. I checked the walking and cycling options — and they too offer the option of adding multiple destinations, so it’s a bit odd that you can not plan out a multi-destination trip using public transportation on Google Maps.