if you do want to go deep you can do a couple of things to make up for lack of reach
a) make a lift out portal at the back areas that you can pop out to work on the far items as ken suggested. these are a mess to engineer into things and require you to not have track running over them unless you want to get really fine on your engineering. also then faring the scenery in around the edge of the hatch can be a challenge. also squiggling under and up through can be a real challenge
b) make your layout in the fat areas more modular in two chunks like 24" deep so that with some unclipping/bolting of the front module you can then pull it aside to get to the back module. if you are careful you can figure out a locking mechanism for the front module that would line all your tracks up, but this can be tricky.
c) buy one of those lean out ladders. they are not cheap at $225, but can get you out and over the layout well. curt got one of these for his layout to reach into his back corner. his did not look like it folded up into the smaller size in the ad though. they are bulky, heavy items to move around, but look pretty sturdy from what i saw of curts.
http://www.micromark.com/Topside-Creeper-Step-Ladder-Support-System,8854.htmld) if you are in a basement with good, solid floor rafters overhead you can make your own trapeze type hanger you can clip on to eye rings bolted into the rafters. the trapeze would just need to be a piece of 1x4 with some padding on it. you then stand on a small, sturdy foot ladder and you can then lean over with the trapeze across your chest and sort of hang your top end from it. this sounds pretty ridiculous, but i had fantastic success with it when i built a 3/4" scale model of the monterey bay aquarium years back. it was so huge (like 30' x 30') there were areas in the center we really could not crawl into so i did a trapeze like this from the rafters to hang from above the model. this was a really extreme case and i was pretty much horizontal in it, but it should work fine for a small application where you just need to lean out a bit and dont want to store a huge mechanism to do this.
personally im working on the design of my basement layout and plan to have everything in larger modules that can pretty easily come apart either for major work on the hard to reach areas or for that house move that is bound to come from murphy if you start on a large layout! but im also in the basement so doing some small trapeze points will probably be a good solution for the smaller stuff.
cheers
jeff