Amtrak has shown no evidence of planning to use batteries similar to what is in the Nissan Leaf or Tesla automobiles for the Richmond-DC or New Haven-Springfield portions of the Northeast Regional runs, instead apparently planning to continue to swap between diesel and electric locomotives at DC and New Haven. There should be plenty of battery powered cars on the road that can cover far longer ranges than DC-Richmond or New Haven-Springfield by the time Amtrak starts taking delivery of these locomotives.
In any event, doesn't the train being heavier mean that it can carry a much bigger battery pack than the automobile?
Amtrak's newest diesels produce 4200 horsepower, which is just over 3 megawatts. 40 Roadster battery packs (at 53 kilowatt hours per battery pack, and about a thousand pounds per battery pack) would store roughly 2 megawatt hours (35-40 minutes of the diesel at full throttle), weigh 20 tons, and probably cost something under $2 million at Tesla Model S era pricing. The catenary locomotives Amtrak is buying are over $6.6 million each, so the cost to add batteries sufficient for Richmond or Springfield service might well increase the cost of the locomotive by less than 50%.
Charging such a massive ammount of batteries quickly could potentially be problematic.
Recharging will take longer than refuelling a diesel, but if the recharging can be done in under an hour, that's probably tolerable.