Electric trains in the USA

Which railway lines in the USA are electrified? Which current systems are used on them? On which railways is single phase AC with a frequency below 60 hertz used? (I know e.g. that the railway between New York City and Harrisburg uses AC of 25 hertz) How is the necessary AC for these railways generated? By frequency converters or (also) by special generators in some power stations? Which voltage is used to transmit the power from the converter stations to the substations of the railways? (I saw that on the pylons of the overhead wires of the railway line between Harrisburg and New York City there is also a crossbar for a single phase AC powerline. Which voltage do these lines use?)>