The difference in gauges of Europe’s railways is one of the reasons why it is often misunderstood to mean that there are major differences between all European countries. This is obviously not the case, and this is an opportunity to put things into perspective.
The adoption of other gauges has sometimes been motivated by topography. In the mountains, for example, the importance of having tighter curves meant that a smaller gauge than the UIC standard had to be adopted. This is why Switzerland has a large secondary network that has adopted other gauges, such as the Rhaetian Railway (Rhätische Bahn), the largest metric network in Europe.
Many countries still have secondary lines with different gauges, such as the Train Jaune in France, the Roslagsbanan in north of Stockholm, the FEVE and EuskoTren in Spain or the Circumvesuviana in Naples, to name but a few examples.
The major problem with different gauges is of course the impossibility of simply running trains from one network to another. This is why all the networks with a different gauge live separately, as separate technical islands. They can only meet the standard gauge network in stations that accommodate both gauges. This is the case for example in Chur, Interlaken, Lucerne, Brig or Montreux in Switzerland, where the secondary networks can be connected with a few steps to the platforms of the SBB national network at 1 435mm.
The difference in gauge between two mainline networks becomes problematic when dealing with international traffic. There are 7 European countries in this case, which may seem like a lot, but in reality, only 2 are problematic, due to trade flows.
Finland has a gauge of 1524mm (closer to the Russian gauge of 1520mm), and the orientation of its network is entirely towards Russia, with the exception of a branch in the very north of the country, towards Sweden. The lack of rail traffic to and from Europe is not a major problem for the country, thanks to the sea routes across the Baltic.