Definitions:

1. an unexpected achiever of success noun
   • The winner was a true sleeperno one expected them to get it.
2. an unexpected hit noun
   • that movie was the sleeper of the summer
3. one of the cross braces that support the rails on a railway track noun
   • the British call a railroad tie a sleeper

Synonyms:

• crosstie
• railroad tie
• sleeper goby
• sleeping car
• slumberer
• tie
• wagon-lit