To use the tunnel, simply drive your truck onto the train by going down the ramp of the open barriers. Your truck does not need to be fully on the train, just the cab needs to be over the icon. Drivers in the UK and France will get the achievement Choo-Choo the first time they use the Channel Tunnel.
A ride through the Channel Tunnel costs €300/£240 and takes 35 minutes. The crossing time is accurate in both the game and real-life and is the fastest way to transport vehicles to and from the UK and France.