You are getting into a similar size range as the NorthWest Trunk Lines model railroad. The NWTL is an irregularly shaped 3200 sq. ft. layout covering an area that's 45' x 92' at it's extremes. The layout has been operational for over five years running DCS and TMCC/Legacy with no signal problems on either system. Like the layout you are considering, the NWTL is in a basement and has several concrete walls that obstruct direct line of sight for the remote<->TIU radio.
I plan to have two or three Z-4000s with 2-3 TIUs to control the layout.
Plan for more TIU's. For best results, plan on using at least five TIU's. You want to keep the wire runs from the TIU to the track short for best track signal quality.
Would it be better to separate the TIUs in the main train room and locate one closer to each end?
Yes. The best strategy is break your layout electrically into smaller sub-layouts. Each TIU will supply DCS signal to an area of layout that stretches about 20-30 ft. from the TIU. The TIU should be centrally located in that sub-area. You will likely end up with TIU's at the far ends of the layout and in the middle.
If I ultimately have trains circling the entire basement, at least 1 TIU will be more than 50' from a DCS remote sending signals for a train which could enter a new TIU's block. What will happen when a train running off a TIU signal from the DCS remote enters a distant block and switches to another TIU which had not previously received the remote's signals?
The train will continue to follow the last command it received. If the train is running 25 smph, it will keep running at 25 smph. Set all TIU's to "Super Mode" in all your remotes and the trains will run seamlessly from one TIU to the next.
This returns to my initial question: For a train circling a large layout where at least 1 TIU will not receive a remote's signal at some time, will DCS still work well?
Yes, it will work very well if configured properly.
In other words, do all TIUs on a layout need to communicate with the DCS remote at all times?
My other questions regard AIU placement: What is the maximum cable length for the wires from the TIU to the first AIU, then all subsequent AIUs connected sequentially to the TIU? Would the AIUs work well with a total of 29-36' of connection cable linking them?
Yes, the AIU will function well with 30+ ft. of cable in the daisy chain. On the NWTL one of the four AIU daisy chains is over 100 ft. long. HOWEVER, it makes a big difference what ports on the AIU's are in use. If you mainly run switches and only drive a couple of the accessory ports you can get away with very long wire runs. This is because the switch control relays trigger one at a time. You are only drawing enough current through the cable to close one relay. But the accessory relays can all be closed at the same time, drawing up to 50 times as much current. If you run a lot of accessories off your AIU's you will want to modify the daisy chain cable to make more current available towards the end of a long cable run.
Since the main train room is 92' long, I think the TIUs will work better if mounted high, ideally on the central ceiling (9 1/2' high) in the main train room. I would prefer to mount the AIUs under the train table, which would require the initial TIU-AIU connection cable to be 25-30' long. I would mount AIUs #2-5 next to AIU #1, so subsequent connection cables would only be 12-18" long. Would the AIUs work well with a total of 29-36' of connection cable linking them?
You are noticing that you have competing objectives. You are thinking of putting the TIU's high in the ceiling to get better radio reception. But that means your track power runs and AIU cables will be much longer. The best way to solve this is to put the TIU in the best place for track signal and the AIU's; probably under the benchwork. You can then modify the TIU's antenna so it can be remote located in the ceiling. On the NWTL, all seven of the TIU's have had the stock internal antenna removed and been replaced by a shielded lead to a BNC bulkhead on the side of the TIU case. We then connect a 50Ω coax cable and run it into the ceiling where we mount a high gain 900MHz antenna. This gives you the best of both worlds: excellent track signal and excellent radio communications. A couple of the TIU's are even outfitted with multiple antennas to cope with concrete obstructions. For anyone who is interested in remote locating your TIU antenna you can contact me off the forum. My crew and I have modified numerous TIU's for other forum members.
Another benefit of this configuration is that you can afford to run longer AIU cables away from a central control out around the layout. You can save literally thousands of feet of wire by locating the AIU's near the switches