This game would use the main mechanic of Ticket to Ride, which is getting from destination to destination. It would add upon this by having cargo you need to deliver.
How this would work is that the train line you create isn’t permanent and resets after you complete a destination and you have to try to get as many cargo orders complete before the cargo cards run out. This maybe would limit each person maybe having two cards at most?
Difficulty could be added in having more types of train cards necessary to complete a route, as in, a long route needs 3 green, 2 blue, 1 red, to complete instead of the current x of a certain color. This could in turn add a trading mechanic especially if there are a lot more colors to have to collect. Another difficulty that could be added is a maximum card hand, or limiting the amount of double lines between certain cities.
Additionally this could be melded with certain Settlers of Catan mechanics where each person gets a base of operation and receives a certain color each turn. That could add more of a reason to trade since each player would have too much of a certain color while needing something else.
Since the goal of this game would be fast route completion it could help to have more than one thing a player can do per turn, such as trading and then action to stop this game from being too long. Because if there’s one thing I learned playing Ticket to Ride, it’s that despite most people taking their single action per turn quickly some people can still take forever. And it has helped with those players in the past, to let them do more than one thing per turn so they don’t spent half an hour weighing the benefits of their single action. But then, some people take forever regardless of restrictions.