I am a fan of CP Rail a bit more than CN Rail. However I'm in a situation where I would have to realize a capital gain to sell CN Rail and move it to CP Rail, so I haven't really done so yet.
I do have some realized losses from other positions that will offset most of my gains, so it may be something I decide to do in the future.
As mentioned, there are a million reasons to sell a stock, but only one reason to buy. My tax situation has stopped me from swapping CNR for CP for quite a bit.
However, considering CP's expansion due to the Kansas City acquisition, holding both railways is a solid option as well. They have lots of individual exposure and owning both captures the vast majority of North American railways.