The first passenger trains are set to pass through the Gotthard Base Tunnel on December 11 – but how fast will they be travelling? Greater wind resistance than expected is slowing them down and threatening to play havoc with the timetable.

Instead of the planned 200km/h, a physical phenomenon apparently overlooked by engineers means the trains running through the longest rail tunnel in the world will have a top speed of only 160-180km/h, according to a report in the NZZ am Sonntag.

The problem is air columns. Out in the open these air flows created by every moving body can easily escape sideways or upwards. In a tunnel, there is not much room – especially in the 57km Gotthard, which for economic reasons was built almost 10% narrower than the Lötschberg Tunnel for example.

The air columns must therefore be moved along both one-way tunnels by the trains – which requires a lot of energy. “In this case the trains act like pistons in a bike pump,” an engineer told the NZZ am Sonntag.

The problem is exacerbated by slow goods trains, which also use the tunnel but have a top speed of only 100km/h. These take 41 minutes to get through the tunnel compared with around 19 minutes for passenger trains.

Logistics

In theory, the Gotthard timetable is a thing of logistical beauty. Most of the time five trains are travelling in each direction: two goods trains travelling quite close together, one passenger train and two more goods trains. The fast passenger train enters the tunnel just ahead of two goods trains, catches up with the two goods trains ahead of it and leaves the tunnel just behind them. This system, which has a 30-minute rhythm, fits perfectly into Switzerland’s scheduled timetable.

In practice, however, the four goods trains slow down the moving air packet by 100km/h – the difference between the trains’ speeds.

Based on non-representative test runs, engineers reckon 10%-20% will be knocked off the passenger trains’ top speed. More test runs, which will be technically representative, are set to begin on Monday.

If the slower times are confirmed, the timetable will have to be rewritten with fewer than 100 days until the first passengers climb aboard.

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