Japan Railways's infamous bullet trains Hokkaido and Sagawa Express can only run at 160km/hr  between Tokyo and the northern city of Sapporo while they can normally ride at 300km/hr.

They say "The biggest barrier to faster speed is the Seikan Tunnel, the 53 km underwater railway tunnel that connects Hokkaido with the main Japanese island of Honshu".But why? Is is the aerodynamics of the tunnel? No. It is Onions and Potatoes. 

This is not a new year joke. Read it for yourself

https://asia.nikkei.com/Business/Transportation/Japan-bullet-train-to-carry-seafood-for-faster-tunnel-speed