this great document explaining the differences between Amtrak and VIA Rail HEP (Head End Power). HEP is how the electricity generated by the Head End locomotives Power the rest of the train.
The essential difference is that Amtrak's HEP is one circuit from one end of the train to another, while VIA's is split into two circuits that alternate between cars. I believe only one Amtrak locomotive supplies HEP, whereas VIA's design allows two locomotives to supply HEP (one for each circuit) with automatic fail-over to the other loco if necessary. VIA's design is more complex but more robust as well, suiting our harsher climates where loss of heat can be dangerous in winter!
The document is on GN Trains' web site. GN Trains rebuilds private passenger cars so the intricacies of HEP are important to them. They have a whole page of interesting documentation.