HEP 2 are old Budd Stainless Steel cars from the 1950s (some former Amtrak) and rebuilt in the 90s with HEP (Head End Power) for heat and light. They all have 2+2 seating. The "2" indicates they were part of the second HEP rebuilding program.....the first was the rebuilding of the cars usually found on the Canadian and other non corridor trains.

REN are the Renaissance Cars (Nightstar) with 2+1 seating.
The other corridor equipment is LRC, built new in the early 1980s. They also have 2+2 seating.

If booking online, the computer selects the seat but just call the toll-free number to have it changed to the seat you want. I’ve never been refused a change if it’s available.

Link to some car diagrams and 360 degree photos here:

http://www.viarail.ca/en/about-via-rail/our-fleet