In the aftermath of the Minutemen ending, the Minutemen set up checkpoints across the Commonwealth around small clusters of Armored Personnel Carriers. What I'm wondering is, are the APCs there before you complete main questline? Because if they're not, it means the Minutemen somehow have a whole fleet of working armored vehicles. Anyone care to check?

they're already there. most of the "checkpoints" you defend post game are pre-war military checkpoints.

makes finding power armor easy

Ah, okay, thanks.

Maybe they're trying to fix them up? I don't know. It would be pretty cool though.

and on the Institute ending Synths are "defending" these checkpoints,and BOS ending BOS are defending the checkpoints. Don't know about Railroad

Yeah, the checkpoints are good places to find fusion cores and power armour pieces (even tells you this on one of the loading screens). Don't know why the factions would defend them at endgame though, I'd rather they defend my damned settlements lol

I didn't play post-ending on my first character, but I can confirm at least 1 APC showed up, one at Weston Water Treatment Plant. During my first playthrough, I ran through there and noticed this APC because there was a Brotherhood Knight next to it all the time. On my current playthrough, I'm at the Jewel of the Commonwealth quest. I already went to the Weston Water Treatment Plant and the APC isn't there, so I guess it's possible that some of them appear at different points of the story, I don't know how many though, if more than the one I noticed.

Comparison:

http://i.imgur.com/gBtpu5M.jpg

http://i.imgur.com/8w7Pdrn.jpg