The typical diesel-electric locomotives cost around two million dollars a piece whereas all-electric locomotives cost six million dollars a piece. Amtrak bought 70 all-electric locos from Siemens for $466,000,000 in 2010. There are presently 26,000 diesel locomotives operating in the USA. To switch over would not only include building that many new locomotives and all of the attendant carbon released to do so, but electrifying thousands and thousands of miles of track, and that carbon footprint too. It's impossible to make any kind of switch over to electric from anything without building a lot of new stuff.

The best thing for the environment is not to buy a new Tesla, but to drive your old gasoline car until it falls apart. It's the same with rail, and a lot cheaper. People like new things, so they can't seem to get a grip on that.