Two tunnels, one running North-South and another East-West, measuring a total of 35km built 60m underground. A total of 18 utility buildings with shaft access.
- East-West 1 – Ayer Rajah to Holland Road
- East-West 2 – Holland Road to May Road
- East-West 3 – May Road to Paya Lebar Road
- North-South 1 – Gambas to Mandai
- North-South 2 – Mandai to Ang Mo KioNorth-South 3 – Ang Mo Kio to May Road