The closest I've come to crossing Canada by rail was from Toronto to Vancouver and that was a few years ago.
I can cover [most of*] the rest of the country for you. Made many trips out east, on both the CN and CP lines. Not sure if VIA ever used the CP line, but I have taken VIA over the CN line as well.
*I spent an entire summer in Newfoundland while there was still a train, however we got there by air and around the rock by car. I spent several weeks with my aunt who lived by the tracks in a small remote village.
I have also driven from St. Johns to Victoria
Likewise, but not in one trip. Countless trips from Montreal east, and 4 from Toronto out west (all to BC, but only 2 to the island) by different routes (ok, 3.5 because one trip we returned through the states).
What's it like going cross Canada on passenger rail? How expensive?
Expensive is relative. Do you want to go coach, or get sleeper accommodation. It could run you well into the thousands, far more expensive than first class air, if you want to. I think Omni's several hundred must be many years ago, I think even coach is quite expensive now. I believe there are rail passes available if you want to spend some time and stop along the way, they used to be less expensive if you purchased them out of the country but I haven't checked into them recently. The carrier is called
Via Rail.