This situation has occurred largely for historical reasons, because technical progress has enabled more economical systems to be introduced wherever full interoperability with earlier electrification was not then required. Each operator chose what it considered to be the most suitable for its own application, and the government typically had a 'laissez-faire' attitude.
And the technology was evolving, so that the best choice at one time might not be the best several decades later. Finally, it the investment needed is incremental to build multi-system motive power for use in interworking than to re-electrify on another system with all its implications for service patterns and safety systems.
Until quite recently, DC traction motors were widely preferred, but because the public power distribution network operates at 50 Hz AC various different arrangements for converting it to DC and delivering it to the motive power were adopted. Until recently, the only practical alternative to the DC motor was the low-frequency AC motor, and this was the basis of the strange but widely-adopted system in which the catenary is supplied at 15kV 16 2/3Hz.
Nowadays, with a strong policy (and public funding) for integration within the EU, has the position changed significantly. Instead of the former public support for 'national' standards - incompatibility was sometimes seen as an advantage, protecting local manufacturers and operators from foreign competition - we have investment concentrated on pan-european standardisation. But there is also commercial pressure for 'lowest-common-denominator' solutions, eg the 66 diesel loco with a profile that lets it run almost anywhere. During the following years we might see a situation with each rail service operator free to invest in whatever their shareholders see fit, while the infrastructure providers collaborate in creating a 'seamless pan-european network' for them to run on.
Here is an overview of the four systems that are most widely used in Europe today, which the railways will have to cope with for some more years because of the reasons mentioned above. Meanwhile numerous multi-system electric locomotives and EMUs are being produced, and passenger vehicles intended for international running are equipped to take 'hotel' power as supplied by the locomotive at 1000, 1500 or 3000V. On the other hand, as can be seen from the » maps, several countries with extensive DC networks have adopted 25kV 50Hz for new electrification, particularly in the case of high-speed lines.