[jcranmer]

There are a few reasons for the mass abandonment of railroad track. Probably the biggest (at least in the US) is railroad consolidation--we've gone from ~100 Class I railroads to 7 (8 if you include Ferromex in Mexico). This consolidation means you don't need parallel mainlines. Additionally, train traffic has generally shifted from serving individual cars to customers along the tracks to a heavier emphasis on delivering to consolidated freight stations (with last-mile delivery handled by local truck), which means the utility of tracks in the suburbs is lessened.

[iguy]

In 1936 the UK was already consolidated down to 4 companies (IIRC). The big abandonment was deliberate government policy in the 60s, and the logic was exactly last-mile truck (or bus) delivery with rail for the trunk. But it turned out that once you put things on a truck, they tended to stay there the whole way, since changing modes has significant friction and the country isn't vast.