Daniel, You've gotten plenty of very good advice so far, so if this is helpful additionally, here's is a view of my set-up that has been powering the whole layout - 4 old ZW's and one newer type - since 1995. None of them have ever been serviced or repaired. I just found them randomly at shows and at my LHS. Plus, I bought an extra ZW I keep stored under the layout, just in case.
The five of them run ten trains, only two trains per ZW, using just the larger handles, except that one of the ZW's additionally powers a separate loop for a trolley, using a smaller arm; another one powers a Rotary Beacon on a smaller handle, too.
One thing I did discover, though, even with spreading my trains out among the five ZW's, which is that whenever I have tried to run two passenger trains (scale cars and scale steamers) in-tandem on the same track, the power would sputter and very quickly blink out. I soon learned that all those lights were too much for one handle.
If it's further helpful for you to know, I've run trains by Lionel (my preferred manf. due to the convenience of TMCC and cruise-control), Weaver; MTH; Williams; K-Line; Atlas; 3rdRail with those five ZW's. However, nowadays, I run strictly the TMCC Lionel engines, esp. since visits to our layout by guests this Holiday season will be aplenty. The TMCC is best for my needs, as is the obvious ZW commitment I have made, all of which affords me as relatively trouble-free a Holiday season as I can get with the house full of guests.
Frank M.