The construction of the tunnel was carried out by the Hamburg engineering company Brandt & Brandau, of Karl Brandau and Alfred Brandt.
 On average, 3,000 people a day worked on the site.
They were mostly Italians, who suffered under very poor working conditions: 67 workers were killed in accidents; many died later of diseases.
During the work, there were strikes, which led to the intervention of vigilantes and the Swiss army.

With up to 2,150 m (7,054 ft) of rock over the tunnel, temperatures of up to 42 °C (108 °F) were expected and a new building method was developed.
In addition to the single-line main tunnel, a parallel tunnel was built, with the tunnel centres separated by 17 m (56 ft), through which pipes supplied fresh air to the builders in the main tunnel.
It was envisaged that the parallel tunnel could be upgraded to a second running tunnel when required.
The first Simplon Tunnel (19,803 m (64,970 ft) in length) was built almost straight, with only short curves at the two tunnel portals.

On 24 February 1905, the two halves of the tunnel came together.
They were out of alignment by only 202 mm (8 in) horizontally and 87 mm (3.4 in) vertically.
Construction time was ​7 ¹⁄₂ years, rather than ​5 ¹⁄₂ years, due to problems such as water inflows and strikes.
The construction of the tunnel was carried out by the Hamburg engineering company Brandt & Brandau, of Karl Brandau and Alfred Brandt.  On average, 3,000 people a day worked on the site. They were mostly Italians, who suffered under very poor working conditions: 67 workers were killed in accidents; many died later of diseases. During the work, there were strikes, which led to the intervention of vigilantes and the Swiss army.
With up to 2,150 m (7,054 ft) of rock over the tunnel, temperatures of up to 42 °C (108 °F) were expected and a new building method was developed. In addition to the single-line main tunnel, a parallel tunnel was built, with the tunnel centres separated by 17 m (56 ft), through which pipes supplied fresh air to the builders in the main tunnel. It was envisaged that the parallel tunnel could be upgraded to a second running tunnel when required. The first Simplon Tunnel (19,803 m (64,970 ft) in length) was built almost straight, with only short curves at the two tunnel portals.
On 24 February 1905, the two halves of the tunnel came together. They were out of alignment by only 202 mm (8 in) horizontally and 87 mm (3.4 in) vertically. Construction time was ​7 ¹⁄₂ years, rather than ​5 ¹⁄₂ years, due to problems such as water inflows and strikes.
Simplon Tunnel