1. Go forward until you reach the end of the alley, then turn left.
2. Examine the bird cage. There's a ticket inside. Take it.
3. Go to the other side of the alley until you hit a padlocked gate.
4. Face this gate and turn right to see a door with a symbol on it.
5. Click on the door to find the secret ticket machine.
This unlocks the achievement Ticket Machine.