Moving through blocks

While there is 20 amps of current "available", the locomotive will only use what it needs.  I don't see any problem with how you're wired, and that's the way I'd do it as well.

Thank you, John.  I have a situation similar to Jeff's.  Command (TMCC), 18 volts, but 4 power districts make up my mainline.  All 4 mainline power districts are powered by the same PowerHouse 180 brick.  The layout is wired such that each district could be supplied by their own separate 180 if I actually needed that much juice.  All yards (4 other power districts) are supplied by a second PowerHouse 180 brick. 

At one time, I had two different power sources, but I replaced them with the identical bricks.

George

 What if only one brick trips it's breaker or is not on. A running train enters the block that has no power. In that instant the power is drawn by anything on those rails, thru the engine spanning the block.

 Maybe why so many things happen to our trains just trying to run a lap?

I killed the power to my engine yard to make sure it didn't interfere with signal running a very big consist. A switch was thrown wrong and the train entered that siding. About a dozen engines sprang to life!

The small difference in voltage of the bricks is not significant.  While it seems that shorting the one that reads 18V to the one that reads 19V would result in excessive current, reality is that it doesn't.  The two will actually "meet in the middle", and very little current will result from the mismatch.  I've actually tried this with a PH180 and a PH135.  The 180 reads 18V, the 135 comes in at 20V.  When I wire them in parallel, the current between them is around .1 amp, and the reading is a bit over 19 volts for the common output combined.

If you have widely divergent voltages on the power districts, that's a different matter.