This looks very readable!

Note it's not needed to use TrimEnd() after every symbol check, it's enough to do it once per text line (though not sure which is faster). For style, I would suggest using a different name for comments (as it doesn't contain just comments), maybe lines.

Maybe for good practice, 26 could be defined as const int alphabetCount = 26; at the top. This would avoid the higher maintenance costs of a magic number.