From the rules:
To claim a Ferry Route, a player must play a Locomotive card for each Locomotive symbol on the route, and the usual set of cards of the proper color for the remaining spaces of that Ferry Route.
This sounds like if I had a 6 train span with two of these being Locomotives I could play 2, 3, 4, 5, or 6 Locomotives with the rest being the same color. Because the 'usual set' of cards can include a wild.
However all other places in the manual show multiple examples of wilds factoring in. This one does not. It only shows one example where wilds are not used. The caption on the photo states (emphasis mine):
Claiming the Ferry Route from Smyrna to Palermo requires four Train cards of any one color and two Locomotives.
This makes it sound like Ferries are one example where Locomotives are not wild.