WordNet sense 1 (a rester who is sleeping):

slumberer

WordNet sense 2 (one of the cross braces that support the rails on a railway track):

tie, crosstie, railroad tie

WordNet sense 3 (a passenger car that has berths for sleeping):

wagon-lit, sleeping car

WordNet sense 4 (tropical fish that resembles a goby and rests quietly on the bottom in shallow water):

sleeper goby