Because if it is a significant amount, say, 15 to 30 percent of their gross revenues, then all the Feds have to do to have a first-class rail system up and running by 2010 is to simply rescind all their taxes.
The property taxes are paid to states, counties and localities, and those can be nasty. I don't know if the federales have the authority to rescind state, county and local taxes.
Of course, no highway pays property taxes, which only adds to the tilted playing field.