"The Interstate Commerce Act of 1887, establishing the Interstate Commerce Commission (ICC) had been an early effort to regulate the industry; but over the years, the courts had sharply limited its influence." - wiki

Would the Interstate Commerce Act of 1887 be considered a policy for the "Square Deal" even though it wasn't Roosevelt who passed it?

I would think yes since it's a course of action taken.

4 answers

but I'm not sure if the Act was a key policy for the "Square Deal".
Yes, it was. See the Hepburn Act.

http://www.presidentprofiles.com/Grant-Eisenhower/Theodore-Roosevelt-The-ripening-of-the-square-deal.html

And, you can search on Hepburn Act. Before that, the ICC was very restricted on actual control of commerce.
So, the policies would be the Interstate Commerce Act of 1887, the Interstate Commerce Commission, Hepburn Act, Sherman Antitrust Act ... and more
yes.