I looked up a trip from A to B, and want to keep it "bookmarked", and search for another trip from B to C at another time/date.
I'd like to save a trip that I've found for easy access later.
I think that a simple pattern is to allow adding them to favourites and showing them in a subsection of the "Fave" tab.
Assuming that planning a route is deterministic (which I believe it is), then saving all the parameters (including From/To) should work.
That's a great idea - and would be very helpful for route planning in comparing potential routes (especially once datetime picker is merged as well).
And yes routes are deterministic (given that GTFS data is not updated). So same request should always produce the same result given the same input params.