The 123 subway station at Penn Station finally has B41. Its from the Transit wireless DAS but in the past it only had B25 LTE. Nice to see more B41 lighting up underground. I'm going to check the ACE side sometime this week as well.
I'd just wonder how long it would take to get the networks to mesh. How integrated would they need to be to have seemless handoffs, especially with voice. Then how long will this integration take to get to that point. That's if they even integrate networks. Maybe they could say just point Sprint's equipment to TMobile's servers, push some updates and be integrated. Not knowing a thing about how their networks work, this is the grey area and the area I'd be concerned with.
For data, I can see just open up bands to each others customers, retransmit requests/packets if hopping networks. But not sure that will go well for VoLTE. Plus there could be a lot of lost packets which wouldn't be good for a lot of services. I'd imagine at first say as a Sprint customer I would be able to use the TMobile network, but when the phone scans it would scan for Sprint channels first then scan for TMobile if it can't find Sprint.