Longest <i>railroad</i> tunnel. There exist longer tunnels that are also rather interesting reads:<p><a href="https://en.wikipedia.org/wiki/New_York_City_Water_Tunnel_No._3" rel="nofollow">https://en.wikipedia.org/wiki/New_York_City_Water_Tunnel_No....</a>
Linked article light on details. Far more info on Wikipedia: <a href="https://en.wikipedia.org/wiki/Gotthard_Base_Tunnel" rel="nofollow">https://en.wikipedia.org/wiki/Gotthard_Base_Tunnel</a><p>EDIT: My link was bad. Thanks, zawaideh