I am looking for the title/author of a book I read about 25 years ago.
 It was a paperback, romantic suspense genre, that was donated to our
high school library, so it may well have been written several years
before that.

It was set during or shortly after World War II.  It features a young
woman travelling home from college, I believe during the Christmas
holidays (at least it was definitely winter).  She is travelling via
train, and encounters a serviceman (potentially scarred/disfigured?). 
They become involved in foiling a plot by some bad guys, and fall in
love.

I believe that the author's first name might have been Mary, and that
the last name might begin with Mc or Mac - but I'm not very sure.