I'm going to attempt a bit of an overview of this subject, hopefully published over the next 3 to 4 days!
The mechanics of modal shift part one
Modal shift in transport has quite a history. From the horse to the horse and carriage through the growth of canals, then railways, then the use of roads and aircraft. Most of the change has been over the last two centuries, and has quite clearly been powered by the easy availablity of first coal, then oil.
Early attempts at adopting the steam locomotive for road use were failures because roads simply weren't good enough. But steam locomotives were perfect for railways, and the rail network quickly expanded, killing off a lot of the canal trade in the process. This was the first big modal shift of the industrial era.
The discovery of oil (or rediscovery to be precise) quickly led to the creation of the internal combustion engine. As more cars were produced the road network was modernised, creating a virtuous circle of development. Eventually the car challenged the railways, at least for part of the share of traffic, particularly passenger travel and some of the fiddly freight traffic.
Despite the railways' fantastic contribution during world war two post-war governments in the UK began to favour road over rail, partly because of the huge cost of restoring the rail network after war damage and wear and tear which arose from the very heavy wartime traffic. The railways were nationalized on 1 January 1948, releasing the Big Four companies from the obligation of investment.
All the time this was happening more and more oil was being discovered, petrol was generally cheap and used as if it was an infinite resource. Investment decisions on creation of new transport infrastructure seemed to also imagine that cheap oil would last forever, again as if it was an infinite resource. Motorways were constructed and railways began to be closed in a wholesale manner, particularly after the Beeching Report was published in 1963. The Beeching Report was fatally flawed in two ways, it treated railways differently from roads, requiring railways to be 'economically viable' whereas roads, no matter how remote and little used they were, were treated as a social necessity, with no need to be economically viable. The second big error was to not take into account the limited lifespan of oil. Whilst the railways were now using oil to fuel its new diesel locomotives, replacing steam that used home mined (but equally finite) coal, railways were not tied to one energy source or one delivery system the way roads were.
The effect of the Beeching Report was the wholesale destruction of the rail network, not just through closure of lines (including the S&D) but the closure of hundreds of stations on lines that stayed open and the diversion of freight from rail to road.