Ok, regarding the area near the first choke point, this is the first idea that popped into my head. The goal is to make it a little narrower to force more intense action here since it's relatively spread out right now. The first change is on the track side...
Obviously my building sucks, but you get the idea... something to narrow the track side just a little bit. This could also be a bump-out or prop on the other side of the track, which could also make for an excellent sentry location if it were buildable.
Inside the building is the only part of the map I find kind of boring. It gets loaded with snipers and spam and doesn't seem to generate any interesting gameplay other than "fall down/move up/shoot". I think reducing the length would help protect the room on the defender's side a bit, plus it would give the attackers a good forward base area to push that choke point. This is the first idea I came up with...
Basically, you change one of the doors to a window that cannot be shot through (the upper door) and raise the other one up (the lower door) so the attackers can drop out to attack. The floor in that room would have to be extended over to the wall and you'd obviously need some detailing to fill in the big space around the window.
Here's another sketch from the other side, which includes some sort of props to allow defenders access to the room. Obviously it's still hard for them to get in, so it works similar to a 1-way gate except slightly less defensible. Also note that snipers could still hit any defenders moving across the far end, so you're not completely eliminating that gameplay... just reducing it.
Finally, you need an arrow here. Even though the spawn is aimed in the right direction, some people still felt the need to run out the wrong way.
As always, if my ideas suck, disregard them. I do think that first section needs something more to make it more interesting, but I kind of struggled with ideas for it.