You are using absolute paths, but /mysite/
isn't the document root, so that won't work. Remove the leading slashes to use relative paths and you should be good.
RewriteRule ^article/([0-9]+) mainarticle?article_id=$1 [L]
As it's an internal redirect you can also safely add the .php
extension in case you know that's the file you are targeting, this would avoid an additional rewrite for the missing extension.
... mainarticle.php?article_id=$1 [L]