Your transportations saved in the Library can be used in any trip.
How to Use Transport Elements in Your Trip
Go to Trips, select a trip and open the Itinerary tab
Navigate to the "Day by Day" step
Add a transportation
Start typing the name of your saved element, matching suggestions appear as you type
Or filter by Departure, Arrival or Type
Click the transport element you would like to insert
Make any changes you need in the element details
Save your trip