We don't really know enough about that to call it a real replacement.
Magical Express, you checked your luggage in at the airport, and didn't have to think about it till they showed up in your room at the resort. You got on a Bus that went to your resort - possibly stopping at other Disney resorts on the way, but you got on a bus and a short time later you were at your hotel, generally with cast members welcoming you to your stay when they saw you get off. When it was time to leave, you checked your bags in at the resort, and again didn't have to think about them till you got to your home airport. No lugging them on buses, carrying them in, nothing.
All we know about the train at this point is that it will stop at Disney Springs. Nothing else. Luggage transport? Might be managing that yourself, lugging it on a train, kids and strollers in tow, transferring all of that to a resort bus at Disney Springs. Which is not a terribly convenient location to a lot of the resorts. And that Train isn't a Disney train, it's just gonna be a generic Brightline train with a stop at Disney Springs. My sister laughed at me and said "Man that's like every day life here in NYC." (she lives there we don't) And yeah that's true but is every day life what you want when spending thousands on a vacation?
Magical Express had a lot going for it. Not everyone liked relying on it, and I get that. But for a lot of us it was the perfect kick off to the vacation, and felt like we were entering that Disney bubble where we could sit back, relax, and let Disney do it's thing.
We did a rental car one year from Tampa when the flights in to MCO were kinda meh. Drove in to Animal Kingdom Lodge. it was fairly anti-climactic by comparison, and the car rental felt like such a hassle after years of DME.
I know not everyone gets it but.. yeah, Magical Express really was a big part of what made Disney Vacations special, convenient and relaxing.