These are either companies which own railroads, or own railroad businesses. Genesee & Wyoming owns quite a number of US Short-Line Railroads. Rail America has similar holdings.
The US Signalling manufactureres, GRS, General Railway Signal and USS, United States Signal are no longer in business. They were purchassed by Alstom.
The once mighty EMD (Electro-Motive Division) of GM (General Motors) has been sold off to a private concession, Electro-Motive Diesel.
Most of these firms either design or build or maintain railroads or the equipment needed to maintain the railroad.