Puts things in context, eh?
Heavy rail remained remarkably constant across all time periods, though. In many cases, for many destinations, heavy rail transit is simply better than the personal automobile.
One issue I have with this graph is that it lumps 'light rail' in with trolley. They aren't quite the same animal. Trolleys are uniformly street-running, in mixed traffic. What APTA calls 'light rail' (post 1980) typically uses that type of Right of Way for only a portion of it's running-way. Most light rail systems use a mix of separated, exclusive, and mixed. A portion of freight railway, or the median of a freeway provides separated part, and exclusive and mixed right of way on public roads provides the rest.
As Meyer/Kain/Wohl pointed out in the Urban Transportation problem (50 years ago), using a freight line with railroad gates is functionally equivalent to a heavy rail. The big different between the two is that heavy rail systems use tunnels and light rail doesn't.