avoids the possibility of a polarity imbalance
Just how is that? The locomotive will still cross the same blocks as the passenger cars and it will still be affected by out of phase transformers.
Fred
1) You won't have to alternate the direction of diodes in each car or remember which cars have which diodes if you don't run the whole set(the polarity issue).
2) The lightweight wiring inside the passenger car is no longer carrying, potentially, the electrical load of an entire train, or whatever might be track powered(possibly 10-15 amps depending on the layout and equipment) in the adjacent block, if it's two pickups are bridging the two blocks(the current/amperage erratic/excessive speed issues).
3) The OP said nothing about a phasing problem - only the voltage differential passing a train from one block to another. With long passenger cars and collectors on each truck, the train will behave quite erratically if the voltages don't nearly match. The loco, with heavier wiring and closely spaced collectors, will likely only bridge the blocks once, briefly and quickly. This is still a problem, especially with modern electronics, but nothing like when a whole passenger train continues it along it's whole length, or if you try to park a train with a passenger car bridging the blocks.
