Shransit is the word to look for in Modern Warfare 4's multiplayer menus this weekend. It sounds like a station you missed on the train. In the November 6 notes, Infinity Ward lists Shipment, Shoot House and Transit 213 as the playlist's three maps. The studio also says Hardcore is available through Quickplay.
Shoot House is back with the same three-lane arrangement, this time in a different setting. If you've come back for that map, there are two others in the rotation to get used to. Shipment is the familiar small one. Transit 213 is worth giving a few matches of its own, especially if your first round mostly consists of finding out which corner everyone else already knows.
The fixes you might actually notice
One fix deals with the brief freeze that could occur the first time you edited a loadout in a match. Another tackles loadouts getting into a broken state after Prestiging. Both are easy to miss in a long list of Season 1 additions, but the menu is exactly where you want the game to behave when there's a round in progress.
There's also a fix for completed events failing to count towards the Event Mastery Camo Challenge. Take a look at the counter if you were working on it. The patch notes don't say how every earlier completion is handled, so it would be premature to promise that an old result will appear there.
Shransit is only a small part of the season. If you prefer DMZ, X01 is the underground area to look for beneath Hajin, with new orders and threats. I'd save that for a session with enough time to explore. The three-map playlist has an appeal of its own when you just want a few matches and a chance to remember what your favourite loadout feels like.