For the best SEO underscores can't be used, but when you want to go to a subcategory you get something like this:
How to remove the underscore between the last 4 digits.
And also when you go the a product detail-page on product_info.php (which I change by hand to product-info.php) the URL displays:
How to remove the underscore in products_id
I found several pieces of code in application_top.php, general.php and index.php but when I change the underscore to a minus the links won't work.