Complex sentence: The Gotthard axis, with the new base tunnels at Gotthard, Zimmerberg and Ceneri is being built under contract from the Swiss Federal Government by the company "AlpTransit Gotthard AG".
Split into simpler sentences: The Gotthard axis, with the new base tunnels at Gotthard, Zimmerberg and Ceneri. It is being built under contract from the Swiss Federal Government by the company "AlpTransit Gotthard AG".