The "Crawler" is the boss for the Scream Train level of House of the Dead: Overkill.
At several points throughout the level, Crawler is seen running and attacking from above the train.
Before the actual boss battle, at one point Crawler rips off the roof of a train car and you must fight. After a few shots to his weakpoints, he is temporarily blown away.
First, Crawler will attack by hanging on to the sides of the train and attempting to slash at you. To stop him, you must shoot his attacking claw. This will cause him to momentarily fall off the train.
After this is repeated enough times, Crawler will jump over G and Washington onto the train. At this point, you must shoot Crawler in the face.
After the Battle
After Crawler is defeated, he falls in front of the moving train as G and Washington watch, to which Washington responds by saying it's "icky" and revealing his dislike towards bugs.