The rules of Nordic Countries (found on Days of Wonders’ website) are a bit confusing and I admit having to read them more than once. However, I believe I have understood the edge cases and their implications for your question.
First off, to state the obvious, locomotives can only be played in ferry or tunnel routes or Murmansk–Lieksa (section Train Car Cards):
Locomotive cards can complement or replace the colored cards required to claim a Tunnel or Ferry route, but they can never be used to help claim a regular route.
(Note: Murmansk–Lieksa isn’t explicitly stated here but explained further down)
Section Claiming Routes contains:
The Murmansk-Lieksa route is an exception. On this route, a player can use any four cards (including locomotives) as a substitute for a card of any color.
This is the only mentioning of locomotives in the Claiming Routes section; the following sections are titled either Ferries or Tunnels and thus imply being relevant only to the specific type of route.
In Ferries it says:
Ferries are special routes linking two adjacent cities across a body of water. They are easily identified by the Locomotive icon(s) featured on at least one of the spaces making the route.
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.
- A player can play additional Locomotive cards as a substitute for a color card
- Any three cards can be used as a substitute for a Locomotive card
The rules do distinguish between the required locomotive cards and colour cards. As such, it seems obvious that only actual locomotive cards may be used as a substitute for colour cards and only the required locomotive cards can be replaced with a set of three. It is not immediately clear if you could use four locomotives and six colour cards to buy that six-train ferry route with two explicit locomotives added. What does seem clear is that you cannot substitute a card that should be red by three yellows. (The link in Pat’s answer does not clear up this confusion as far as I can tell.)
As I mentioned, this was exclusive to Ferries. In Tunnels it says:
When attempting to claim a Tunnel route, a player first lays down the number of cards required. Then the 3 top cards from the Train Car card draw pile are turned face-up. For each card revealed whose color matches the color used to claim the Tunnel (including locomotives), an additional card of the same color (or a locomotive) must be played
to successfully claim the Tunnel. See Examples 1 and 2. If the player does not have enough additional Train Car cards of matching color (or does not wish to play them), he may take all his cards back into his hand, and his turn ends. The three Train Car cards revealed for the Tunnel are discarded.
Be aware that with regard to Tunnels:
Locomotives are multi-colored wild cards. As such, any Locomotive card revealed, forces the player to add a Train Car card (of matching color) or a Locomotive from your hand.
If a player exclusively plays Locomotive cards to claim a tunnel route, only additional Locomotive cards drawn from the deck will be considered a match. This means you will not have to worry about a colored card of the Tunnel's color triggering a match! If Locomotive cards appear in the 3 cards drawn for the Tunnel, triggering a match, it can only be fulfilled by playing additional Locomotive cards from your hand. See Example 3.
This section contains no mention of any possible way to substitute locomotives with any other set of cards as locomotives are never required in the first place. It does mention the possibility of colour cards being replaced by locomotive cards as both the corresponding colour and a locomotive trigger additional cost. It also mentions the all-locomotive tunnel in which case only locomotives count.
Thus, in a tunnel’s case a locomotive can substitute for a colour card if present but colour cards can never substitute a locomotive.