This is the best one I've found in the UK:
The site appears to be adding US railroads as well but I don't know how complete it is.
The UK map has lots of layers so for example you can turn on/off disused railroads - we have lots of these, a large number of small lines across the rail network were abandoned in the 'Beeching cuts' in the '60s. There are also layers for the new HS2 high speed railway (including parts that have been cancelled!).