A culvert is an underpass under the railway line. In this way, animals can cross the line. There are wet and dry culverts. A dry culvert is situated above surface water level and can be made suitable for a variety of mammals. A wet culvert is, as the name suggests, built under water, allowing other animals living in the water to pass under the railway line. If the ecological connection has to be broader than the size of a culvert, the HSL may also cross the watercourse by means of a short viaduct. If the watercourse is characteristic or of decisive importance for the surroundings, the watercourse including embankments runs in its entire breadth under the HSL. The embankment ridge has a breadth of one metre.

Both types of culverts are as short as possible. However, if a culvert extends over some length, there are light shafts every 20-30 metres.

In places where the HSL cuts through a natural area and crosses other infrastructure, paths along the railway track enable mammals, reptiles and insects to cross this infrastructure parallel to the HSL. These paths are called parallel wildlife passages. These passages are at least one metre broad, with a maximum length of 50 metres. The base of the wildlife passage consists of 0.3 metres of unfertile soil or vegetation sods; the passage is not covered but open. The access point is in line with a slope or roadside and easily accessible to the animals.