The biggest cause, historically, is the fact the railways were privatised when much of the rest of Europe was electrifying in the inter-war period and receiving

The biggest cause, historically, is the fact the railways were privatised when much of the rest of Europe was electrifying in the inter-war period and receiving little state money, and at the same time were struggling to make any money (a large part of that was the fact they had an obligation as a common carrier to accept any and all freight for a fixed price, from when they were the only way to quickly transport goods long-distances over land). By the time they were nationalised in the 1950s the railways were still in heavy decline and more or less up until the early 2000s all the planning was about a managed decline of the railways, hence the lack of investment, justified by the falling passenger and freight numbers year-on-year.