Lemme give a very rough, but differentiated estimate:

• Branchlines: renew/rebuild 140,000 km @ $1.4 million/km = $200 billion
  
• Suburban railroads/rapid transit: build/rebuild 70 networks (all urban areas > 500,000) of an average 200 km (sum: 14,000 km) @ $14 million/km = $200 billion
  
• Mainlines: renew 100,000 km @ $5 million/km = $500 billion
  
• High-speed lines: new construction of 20,000 km @20 million/km = $400 billion

In addition,

• Light rail: build/rebuild 150 networks (all urban areas > 200,000) of an average 50 km (sum: 7,500 km) @ $26.7 million/km = $200 billion
  
• Subway/metro: build/renew 70 networks (all urban areas > 500,000) of an average 50 km (sum: 3,500 km) @ $282 million/km = $1 trillion

Altogether $2.5 trillion. Could be significantly less if projects are well-managed (taking lessons from Spain), much more if incompetently and in a very corrupt way. That's a helluva' lot of money, then again, it would not have to be spent in a single year. Compare say an annual $250 billion (assuming a ten-year crash programme) to the annual $600 billion for the military and the one-off $700 billion for bank rescue.

_{*Lunatic*, n.
One whose delusions are out of fashion.}