Belgium has the highest railway density of 1 km of railway for every 6.5 sq km of land among the given options.

Railway density refers to the length of railway tracks in a country relative to the land area. It is an important measure to assess a country's transportation infrastructure and connectivity.

Here is a detailed explanation of why Belgium has the highest railway density among the given options:

1. Railway Density Calculation:
- Railway density is calculated by dividing the total length of railway tracks in a country by the land area.
- In this case, the railway density is 1 km of railway for every 6.5 sq km of land.
- The country with the highest railway density will have the smallest value in this ratio.

2. Railway Density in Switzerland:
- Switzerland is known for its efficient and extensive railway network. However, it has a relatively larger land area compared to Belgium.
- The Swiss Alps and mountainous terrain make it challenging to build railway tracks in certain regions.
- Therefore, Switzerland may have a lower railway density compared to Belgium.

3. Railway Density in Denmark:
- Denmark is a relatively small country with mostly flat terrain, which makes it easier to construct railway tracks.
- However, Denmark has a larger land area compared to Belgium.
- Therefore, the railway density in Denmark may be lower than in Belgium.

4. Railway Density in Italy:
- Italy has a diverse landscape, including mountainous regions, which can pose challenges in building railway infrastructure.
- Italy also has a larger land area compared to Belgium.
- Therefore, the railway density in Italy may be lower than in Belgium.

5. Railway Density in Belgium:
- Belgium is a small country with a highly developed railway network.
- Its compact size and relatively flat terrain make it easier to build and maintain railway tracks.
- Belgium has a well-integrated railway system connecting its major cities and also provides international connections.
- As a result, Belgium has the highest railway density among the given options.

In conclusion, Belgium has the highest railway density of 1 km of railway for every 6.5 sq km of land among Switzerland, Denmark, and Italy due to its small size and well-developed railway network.