It's comically (and extremely variably) priced. A trip from DC to NYC and back would be ~$25 in electric costs with a typical electric car versus Amtrak could easily be $300+ though possibly as cheap as $50 if you are flexible to awful hours like depart at 4:30am or something.
the actual cost of a trip between times square and the national mall is about $200 all things considered based on the ~0.80 federal mileage reimbursement rate for 250 miles. that train corridor is overwhelmingly successful as well so the idea that amtrak isn't a good deal is at odds with reality.
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You should factor in the time/stress/wear costs but yes, I've found driving to be significantly cheaper than even the DC Metro most days.