Alternate Route is the 3rd level of the Fun difficulty from the demo disk included with the Lemmings: The Official Companion strategy guide book.
Strategy[]
The first Lemming must dig a hole the moment it's no longer immediately under the trapdoor. (The second digs another hole, separately, if necessary.) Once they get deep enough to trap the others, bash for 100%.
Alternatively, the first two must block the others tightly. Turn the release rate up to 99. Bomb the blockers, starting with the right-hand one. You lose 14 this way, which is still more than enough to pass.