When a player is skating, sometimes unknowingly, into the sights of an oncoming defending player who is about to lay them out with a huge hit.
Often the trolley tracks are located just over the blue line as a player skates horizontally with his head down allowing defenseman or back checking forwards to lay a crushing hit on the unsuspecting puck carrier. It could also occur when a player receives a "suicide pass" as he looks behind to catch the puck.
Scott Stevens was the most famous defenseman to line players up as they tried skating into the zone and ended up in Stevens' trolley tracks. The reference comes from the fact a player like Stevens is a large locomotive or train that's about to run over anything in its path. Stevens famously crushed Eric Lindros, which ended up leading to the end of Lindros' career due to repeated concussions.