When playing Ticket-to-Ride USA, is there a limit to the number of locomotives used to build a route? It seems unfair to create a 3-space-route with one Train Car card and two Locomotive Cards.
No, there is no limit. You could even complete a route using only locomotives. Remember that even though Locomotives are Wild and can be used freely like that, it is harder to collect them. Rather than being able to pick up TWO cards on your turn (i.e. two yellows), when picking up a Locomotive, you can only pick that ONE card. The only way you can collect TWO Locomotives on your turn is if you draw both randomly in succession...steep odds.
Now, in Advanced sets, some Routes REQUIRE one or more Locomotives as part of the total number.
In the original rules, the relevant statement is
Locomotives are Multi-colored and act as a wild card that can be part of any set of cards when claiming a route.
"Any set of cards" means that it may contain more locomotives. Note that it does not mean that it may contain any further combination of colours, as
To claim a route, a player must play a set of cards equal to the number of spaces in the route. A set of cards must be of the same type.
In the rules of Ticket To Ride: Europe this is explained better:
Locomotives can always act as a stand-in for any given color.
and (as an example):
To claim a Blue route that is three spaces long, a player may play any of the following card combinations: three Blue cards; two Blue cards and a Locomotive; one Blue card and two Locomotives; or three Locomotives.