Yes, you will get Dinner, but since the Departure isn't until 7pm, there's no way to know what time your rez will be until you board, but I'd check with the Person @ the Desk in the Metro Lounge when you check in, perhaps they'll make your Dinner Rez there like they do on the Auto Train.??
If not, ask your attendant when you board trainside, they'll know the procedure the Diner LSA uses.
I also suggest that you use a Red Cap from the Lounge, that way you ride on a Cart from the Backdoor of the Lounge to the Platform instead of walking all the way thru the Station and Tunnel to the Platform