If, at some point, you decide to go on a trip to a Very Exciting Destination, where you plan to meet a friend whom you have not seen in two years, and this is to be a day completely without children that has been on your calendar for oh, say, two years, and you bought your Amtrak tickets online and have your bag packed with a whole day's worth of glorious knitting, and you are over the moon because you are about to have more knitting time in one day than you usually get in six months, and you have nursed the baby and made your lunch and are happily bopping your way out the door at 6:20 am in the pouring rain for the hour-and-a-half bus trip to the train station, because you do not drive, well, please take a moment to double-check your bag for your passport. Why? Because it might not occur to you that you would need a passport to take a train two and a half hours to the next state, but if you do not drive and do not carry a driver's license, you are going to need your passport, because otherwise Amtrak will not let you on the train. So bring your passport, because otherwise you will find yourself standing at the train station in tears as the train goes away without you, even though you have already paid for your ticket, because there was not anything like enough time to go home and get your passport and come back in the 45 minutes left before departure, and all the rest of the trains that day will be sold out, but it won't matter because if you even if you did wait four hours for the next train it'd be pointless because you would miss the excitement at your destination and you'd only have a couple of hours before you'd have to turn around and go back. And you will find yourself trying to figure out how to get back home on the bus, because you didn't figure out the schedule that way before you left, and if you are super unlucky you might get onto the totally empty bus and pick the one seat on the bus directly under a leak in the roof (pouring rain, remember) and sit right in a puddle so deep it will actually splash as you sit down in it. And then, instead of spending your day wallowing in wool with friends with similar wool-wallowing inclinations, you will spend it wallowing in self-pity as you fold laundry and clean your house and change dirty diapers. Which, under ordinary circumstances, you would be perfectly happy to do, but having been done out of the trip you were planning, you will be something less than excited about.
So, yeah. Save yourself the grief. Pack the passport.