[byrneseyeview]

private transportation is subsidised much, much more than rail transport as a function of GDP, passenger miles, or population. It isn't even close.

That is surprising. Source? Also, does your source explain why someone would want to know the subsidy per person, mile, $ of GDP, etc., instead of the percentage of spending accounted for by subsidies versus other activities? It seems that the latter measures what you would actually care about -- e.g. the government probably spends more money on retirement (through Social Security) than it does on space flight. But the government spends a higher fraction of the total money spent on space flight than on retirement, so one would argue that space flight is more subsidized.